There are lots of cryptocurrency payment gateways. Each of them have different processing fees and transaction fees. some of them charge 1% to 5% fees on each transaction. which is a huge amount but merchants have to pay this amount. But there are payment gateways who charge the lowest amount like, one payment gateway only charges 0.23% processing fees on transaction. I find it very attractive because merchants will get more from it and it will be more beneficial for them eventually.
It's called a transaction fee by the way, and it plays an important role for merchants who trades large amount of funds, as they tend to spend less and save more on a gateway that charges very low fees compared to gateways that charge very high, but however, there are some gateways/exchanges which I have used which offer zero "transaction fee" when convwrting your Bitcoin into fiat currency such as "Remitano". Because imagine you are to transact $1000, and a 1% fee will be equal to $10, whereas a 0.25% few will be $2.5, which is way lower compared to the first.