Dear Alex Johnson,

When we address someone in a letter, we use a comma after their name.

I'm really sorry about thate mistake that we shouldn't make itI made.

I'm not sure what you're referring to here. I made a correction that I think would make sense with what you might be trying to say.

And I also do know what you are concerned about currently.

Why tThe problem come out is thatstarted because the plate of the solar battery of the batch haves a flaw that doesn't brush on average.

This sentence is a bit confusing for me. I'm not sure what you're trying to say. But "The problem started because..." is a much more natural, less clunky way to start this sentence.

I can sentd you the new solar battery template with no extra charge.

send = present
sent = past

And there is ono need to return the whole light you have received.

There isare two ways of how I sent tothat I can send you the replacement:ship ground shipping or air.

It takes a long time to transport the goods by ship thaground shipping. It requires 25 days, while it justonly takes 5 days by the air onlyair.

I will carry on all thewaive the shipping fee by the air if you choose this way tofor transport.

"Waive" is a good word to use here - it means to remove the cost of something

I can add a discount coupon infor a future order.

last,pPlease confirm you r choosenice and refer the quantity ofo the products that are not good in use so that I can produce it propercess your replacement correctly.

25.12.20

Depending on what country you're writing to/from, there are different formats for dates. In my country (the United States) we would write the date like this: 12-20-2025.

I really sorry about that mistake that, we shouldn't have makde it.

I have changed it to past tense since you are talking about something you shouldn't have done in the past.

I took about the middle "that", and linked the 2 thoughts in your sentence by a comma instead. This can be a good way to link 2 parts of a sentence. It is also ideal to avoid excessive repetition of words within a single sentence.

And I also do knowunderstand what you are concerned about currently.

"Know" IS correct but "understand" may be a better word choice, to show you are sympathising with the recipient of your letter. For example, "I know you" just means you have met a person, but "I understand you" implies to understand them on a deeper level. Understanding implies a deeper level of knowledge that just knowing.

Concerned not because it is past tense but because the word is always written like that, eg. I am concerned, he is concerned, they are concerned [about XYZ issue]. It is the same for words like frustrated or suprised, if you are adding the "to be" verb before it. To use "concern", you would say "I understand your concern" or "I understand you have concerns".

WhyThe reason for the problem come out is that the plate of the solar batteryies of theis batch have a flaw that doesn't brush on average.

"Why the problem come out" isn't something we would say. To explain a "why", you can use "the reason".

Batteries is plural when referring to all the batteries in the batch.

I removed the last part of the sentence as I didn't understand it and it's not needed.

I can sentd you the new solar battery template with no extra charge.

Sent is past tense. For present and future tenses, use send, eg. I can send, I will send, I would sent, vs I have sent

And there is ono need to return the whole light you have received.

Typo.

There isare two ways of how I sent to you the replacement: ship or air.

"There are" for plural/multiple things, "there is" for just one thing.

It takes a long time to transport the goods by ship - that requires 25 days, while it just takes 5 days by the air only.

Either just or only, not both.

You can use a hyphen (-) or comma (,) to join 2 parts of a sentence smoothly.

I will carrytake on all the fees by the air if you choose this way to transport.

You take on fees or expenses, rather than carry.

Fees is typically plural.

To transport feels redundant here.

I can add a discount coupon infor a future order.

Don't forget to use "a" before nouns, since you are talking about a single specific future order.

lLast,ly, please confirm you r choosenice and referspecify the quantity of the products that are not good in useworking so that I can produce itfulfil your order properly.

Always capitalise the first letter of a sentence.

Specify would work better than refer here. Refer typically means to mention something in passing, or vaguely. Specify means to say something exact.

Working is the word you need here, we would not say "good in use". Other options could be functional, operational, or usable.

Orders are fulfilled. Produce (as a verb) usually means making something from scratch, eg. art or food.