I doubt if Bitcoin payment gateways truly have tye lowest transaction fee except when using lightening.
As far as i'm concerned Visa takes 5% from each transaction. 5$ out of each 100$.
5 sat/vbyte is enough for bitcoin tx to be included in next block which is most often below 1$. Means that if you are about to pay >20$ its better for seller to accept bitcoin for this transaction as long as coinsgate/bitpay or any other gateway is not adding much more fee above bitcoin network fee.