For example seller sells something for 1BTC, I withdraw 90%, so 0.9BTC converted to fiat, to seller's bank account.
Is something like that possible?
I'm not sure I understood what you mean. My guess is the 1 BTC is the buyer's money, so the word you are looking for is not withdraw (maybe exchange, then withdraw).
In that scenario you'll have to have an account to an exchange - probably a KYC centralized exchange - and API keys for that account.
The shopping platform will transfer 0.9 BTC to the exchange; your API will sell that 0.9 BTC based on your settings and market price; when your sale was successful you'll withdraw the fiat to a bank account.
This would be the theory, but I've never done that, you should verify and confirm for yourself.
Even more, my not-too-big experience tells that many banks don't like to get money from crypto exchanges. Also I think that I've also seen requirements like the account on the crypto exchange has to be on the same name as the bank account. But maybe I've got that wrong and maybe that's different in case of companies. You have to research though. (Or maybe you'll have to withdraw you your bank account and then transfer to seller's account?).
