What's the most important thing for you if you started to buy things online with bitcoin?
Privacy and a smooth interface. Lightning would be a bonus.
Make sure you generate a new address for each customer for each transaction. I would not be happy using a service which only gives me a single address to pay to.
I've ran in to problems with some services who calculate prices in fiat, and give a price in bitcoin. Most will give a window of something like 15-30 minutes for the transaction to be made, and after that the transaction will be cancelled to protect from price changes. However, there was one I used in the past which wanted the transaction to be confirmed, not just broadcast, within the 15 minute window. Even with a high fee, sometimes that is impossible if there is a longer delay to the next block being found. It was then a total hassle to get them to credit my transaction against a future purchase.
I really like your idea of having your balance within the platform.
In that case, maybe take a look at how Bitrefill works. You can pay the exact amount required when you place an order, but you can also deposit BTC to your account, and then pay from your account balance instantly when placing an order. Sometimes when I use it, to save on generating small change outputs, I'll transfer a whole UTXO to my account and then buy what I need, leaving the extra few bucks in my account. As much as I don't like ever leaving my funds on third party sites, it's only ever a small amount so I don't worry about it, and I'll always spend it in the not too distant future.