My friend sent a picture to me yesterday, of something she said one of her high school classmates asked her to buy it in Japan.

"of something" replaces "it" - and it changes everything after "yesterday" into a restrictive relative clause, so you still get to keep this all as one sentence!! you're welcome >:)

also - your friend didn't buy the picture she sent to you. she bought the thing that was in the picture. so when i added "of something" i changed everything afterwards into a giant kind of adjective clause to describe the "something" that was in the picture. if that makes sense?

I have been working as an optician for over 10 years, so she messaged me and asked for some suggestions.

usually "suggestions" imply advice or recommendations, i think a better word here could be "clarification" or "explanation"

I said that's one famous brand of RGP which is an abbreviation writing of Rigid Gas Permeable lens.

an abbreviation already implies that it's a writing, you don't necessarily need it.

or - if you wanted to use "writing" as a noun and say RGP is a(n alternative) writing of Rigid Gas Permeable - you would then have to make the noun "abbreviation" into an adjective and say:
...RGP which is an abbreviated writing of Rigid Gas Permeable lens.

But it can only be sold by hospitals in China, but I don't know why it can be bought online without a doctor's prescription.

"why" refers to the reasoning behind something. i think it works here, it just means "what is the reason you can only get it in a hospital in China, but online you can get it for any reason?"

if you were to use "how" it would still make sense, but to me the meaning would change. it would go from asking "what's the reason?" to "in what manner/way can someone do this?"

to me, if you had used the word "how" here instead, i would think that buying it online is illegal, and you aren't sure in what way somebody can sneak past the law and still buy them online. but with "why" you are moreso asking for what reason people can buy it online without a prescription with no repurcussion, implying that you personally disagree with it, and you believe it should not be allowed

it's a slight difference in meaning and i think most people wouldn't put any thought into it if they were just speaking casually. and i'm american fwiw :v

My friend sent a picture to me yesterday, s. She said one of her high school classmates asked her to buy it in Japan.

This reads better as two separate sentences.

She asked me what it is and what is it for.

I said that's onea famous brand of RGP which is an abbreviation writing offor Rigid Gas Permeable lens.

But iIt can only be sold by hospitals in China, but I don't know whyhow it can be bought online without a doctor's prescription.

"why" makes sense but we use "how" more often in the US.