The payment processors are only charged a tiny fee, not a percentage of the transaction. Up until a few years ago Stripe, Paypal and others had no issues with refunding the entire amount.
My biggest issue is that they keep all the fees in case of a refund. Sometimes customers just change their mind, and this will cost you. It will cost you a lot if you’re selling high value items.
Yes you are missing something important. Once they've tied the browser ID to your personal account they can track you across all google properties, even the ones that you didn't log into.