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.
Low merchant fees not only give them availability to get more profit and decrease expenses but it also lets them to run promotions like cashback and make their promotions more attractive while still having less expenses than they would have with payment gateway that charges up to 5%.
But the problem is that low fees alone aren't that beneficial for merchants. Still, a lot of people use coinbase payment gateway because it offers coin to fiat convert option and you don't have to run it on your server, i.e. responsibility is on 3rd party for you.
While 0% is a better choice if one has a high transaction volume, still, there are some benefits that some big merchants prefer, that overcome 1% fee difference.