I hadn't eaten souch good tomatoes for a long time.

“Such” and “so” mean more or less the same thing. “Such” is used before the an adjective and noun, like in “such good tomatoes”, while “so” is used after a noun, where you say that the noun is so _____, such as in “the tomatoes are so good”. In a situation like this where you are talking in the past tense, “such good tomatoes” is the better choice.

I used to buy vegetables and fruitfruits and vegetables on an online platform.

Not a grammatical error but people tend to say “fruits and vegetables” instead of “vegetables and fruits”.

Also, after looking at the next sentences, if you still buy them online you could say “I usually buy” instead of “I used to buy”.

It' was very convenient.

If you’re still talking about buying them on an online platform, in the case where you don’t use it anymore, you could say “it was…”

You just need to click some buttons and the things you want will be delivered in front of your door in around thirty minutes.

This is right!

But if you use the “it was very convenient” option, then you could say “I just needed to click some buttons and the things I wanted would be delivered in front of my door in around thirty minutes.” Of course, you could change the “I” and “my” to “you” and “your”. I just thought it would work better with “I” in case the person you’re talking to has never used the online method.

Yesterday I stopped ato enter a vegetable store on my way home.

It made sense originally too, but this is a more natural way of saying it.

Then I bought the best tomatoes of this year there.

“Then” would work better if you didn’t have the sentence about the store to provide context in the middle, but since you did add that sentence then using “there” at the end of the sentence like I suggested would, in my opinion at least, be better.

I decided to join the Grandpas and Grandmas' vegetable team.

This is just a random note but you don’t actually use “I decide” that much. You would use it mainly if you’re talking about how you make decisions. For example “I decide on things very quickly.” When you’re actually talking about a decision that you are making or have made, you would use “I am deciding” if you are in the process of making a decision, or “I decided” if you have already made the decision.

I hadn't eaten souch good tomatoes for a long time.

Thenre I bought the best tomatoes ofthat I have had this year.

Unless you were buying award winning tomates, the latter correction is more appropriate in the context of your piece.

I decided to join the Grandpas and Grandmas' vegetable team.

I bought some njuicey tomatoes last night.

I generally avoid using "nice" because it's a default positive word that often doesn't mean anything xD

I hadn't eaten so gooduch delicious tomatoes forin a long time.

You just need to click somea few buttons and the things you want will be delivered in front ofto your door in around thirty minutes.

Yesterday I stopped to enterby a vegetable store on my way home.

This kind of store is popular among older people who don't like online shopping or who want to pickchoose their vegetables by themselves.

"pick vegetables" can also mean picking them in the fields.

That was when I bought the best tomatoes of thise year.

I decide to join the Ggrandpas' and Ggrandmas' vegetable team.

LOL

I hadn't eaten so good tomatoes forthat good in a long time.

Yesterday I stopped ato enter a vegetable store on my way home.

If it's a small place that only sells fruits and vegetables, we call those fruit stands.