I ordered a seat cover with a silicone pad online three days ago.

Or: "I ordered a silicon padded seat cover online three days ago" <- if and only if the ENTIRE seat cover is padded with silicon. Otherwise, your sentence is mostly correct.

It wasJP (Japan Post) attempted to delivered it yesterday by JP (Japan Post), but no , but since none of my family members wasere at home, so the seat was brought backreturned to athe post office, leaving a notification cardand a note was left at my door since deliveries are expected to be done in person to avoid theft.

It was delivered yesterday by JP (Japan Post) -> JP (Japan Post) attempted to deliver it yesterday: Grammatically this phrase is correct, but contextually it's strange. The seat was NOT delivered, as being "delivered" implies successful completion of the action of "delivering". Therefore, we should specify that there was an *attempted delivery*, and the phrase is reorganized for a more natural way to say it.

no one of my family members [X] -> none of my family members [◎]

but no one of my family members was at home -> but since none of my family members were at home: this correction follows the correction of the phrase earlier in the sentence - if it's said that something's "attempted", and you state the reason why the attempt failed, "since + [reason]" is the most natural way to continue the sentence.

Example:
He attempted to run, but SINCE his leg was broken, he couldn't ◎
He attempted to run, but his leg was broken, SO he couldn't ◎
He attempted to run, but his leg was broken, he couldn't X <- you may hear people say this, and that's because "since" or "so" is implied, just like how particles in Japanese are sometimes omitted by natives because it's implied.

brought back to a post office [X] -> returned to the post office [◎]

leaving a notification card -> and a note was left at my door: more natural way to say it. You don't need to say "at my door" as it is implied through the note the postman left.

Append the next sentence to this sentence.

Basically a delivery should be handed in person to avoid trouble such as theft.

Delivery drivers have to come back forrepeat attempted deliveringes so many times when there is no one at home that it hait's become a mainbig issue in Japan.¶

More natural way to say it. Contextually, "when there is no one at home" is implied from the previous sentences that this is what we're talking about. If we have to include this, the entire sentence has to be reorganized and restructured.

The number ofre's not enough delivery drivers are lacking as the increase of online shoppingfor the amount of people shopping online, and repeated deliveries are worsening the burden.

Complete reorganization and combination of this and the next sentence.

The repeated delivery makes their burden worse.

Okihai (leavconsenting thefor delivery in front of the door with the informed consent in advanceies to be left on the front door) is a solution.

more natural way to say it.

Some residences adopt the okihai-box, which is a kind of very large post to prevent theft.[SEE CORRECTION]

it's confusing to correct this sentence because it the premise seems confused.

Searching online, an okihai seems like a place where packages can be safely dropped. This would mean that it is a sort of lock box bolted to the ground or some other safe area, as an "okihai box" implies the okihai is just another box to put packages in, which doesn't seem right since it would be just as easy to take the box with the package inside. There's three options I can see here, and I've corrected the sentence in all of the assumptions, though I think #3 is what you're trying to go for here:

1. An okihai box is a metal safe bolted to the ground that delivery drivers put a code into, drop the package into, and leave:
- Some residences get an okihai box, which is like a safe bolted to the ground.

2. An okihai box is a zone where there's surveillance, thus making it safe:
- Some residences get an okihai box, which is like a safe zone where packages are dropped and kept under surveillance to prevent theft.

3. Individual kind neighbors decide to retrieve the package for the person, essentially becoming an okihai box:
- Some people decide to pick up their neighbors' packages for them while their away, essentially turning their entire house into an okihai box to prevent theft.

You can choose okihai even thoughelect for an okihai delivery even if you don't have thean okihai-box at home.

even though [X]-> even if [◎]

the okihai-box [X] -> an okihai-box [◎]

You can choose okihai -> You can elect for an okihai delivery: From what it seems like, "okihai" seems like a term that can be used as an adjective just to describe a delivery being left on the front door with advanced consent. If this is the case, we reorganize this phrase.

Most of the cases, delivery products remaIn most cases nothing in front of your door not-stolen until you go home and find them yourselfs stolen, and JP doesn't need to come back again.

Much more concise way to say it, and more natural as well because the rest of the sentence is implied.

The second sentence is combined with this shortened sentence.

JP's delivery staff -> JP: In most cases, delivery drivers of a company are referred to just as the company themselves as the "delivery driver" portion is implied. Specifying it here would be odd as it would imply that there's an exception.

Example:
My package was delivered by Amazon today. ◎
My package was delivered by an Amazon driver today [X] <- as opposed to the package being delivered by.....?

This principle is common everywhere - if you see tomatoes at the grocery store, they're labeled "Tomatoes", not "Genuine Tomatoes", as "Genuine" is implied. To make this example more complete, we can make it more extreme - instead of "Tomatoes", you would probably think the following is comical if you saw this label at a grocery store:

"Absolutely real Tomatoes that are definitely red, ripe, and ready for consumption"

JP's delivery staff members don't come back again.

They'll leave thea notification card, in which you can choose two options; one is to dial to JP and book e which gives you two options: either call JP to organize another delivery date and time, when you're going to be at home; another is to go to the post office and get it yourselfat home, or go pick it up yourself at the post office.

More natural way to say it.

You need to bring your driver's license or Sa social security number card (My Number C) card) to identify that you are youprove your identity.

or Social security number card (My Number Card) -> or a social security (My Number): "number" is implied in social security

to identify that you are you. -> to prove your identity.

Anyways, the post office locatesis in my neighborhood, so I visited there and brought backpicked up the seat cover this morning.

post office locates my neighborhood -> post office is in my neighborhood: wrong verb

so I visited there and brought back the seat cover this morning. -> so I picked up the seat cover this morning.: more natural way to say it

It fit's a perfectly to fit for my bicycle's seat.

More natural way to say it.

I tried to go to my office by the bicyclWhen I biked to my office this morning, I found the seat had become this morninggher, but the silicone pad didn't feel soft at all.

More natural way to say this sentence, and combination with the next sentence with its own correction.

"at all" is added at the end for the emphasis of an expectation that was not met.

I found that the seat position has become a little higher, but I didn't feel the silicone pad was soft.

I need to go for a long ride in order to proof the delivery really works.[SEE CORRECTION]

I have no idea what this is supposed to mean, as you going for a bike ride has no logical implication on whether or not the delivery system set up by JP actually works. I'm going to assume you're trying to say that you need to go for a long ride to prove your seat cover works, which is corrected the following way:

I'm going to ride some more to see if the silicone gets softer.

It was delivered yesterday by JP (Japan Post), but no one of my family members wasere at home, so the seat was brought back to a post office, leaving a notification card.

This change sounds more natural I would think

Delivery drivers have to come back for delivering so many times when there is no one at home that it has become a mainjor issue in Japan.

it is not Japan's main issue I'd think, but it would be a major one.

The number of delivery drivers are lacking asis not keeping up with the increase of online shopping.

I'm not exactly sure what was meant here, but it didn't quite sound correct

Anyway, the post office locatesis in my neighborhood, so I visitedwent there and brought back the seat cover this morning.