I checked my local US bank (Bank of America business account) and for business accounts the processing fee appears to be 0.3% with the first $5,000-20,000 per s

I checked my local US bank (Bank of America business account) and for business accounts the processing fee appears to be 0.3% with the first $5,000-20,000 per statement period exempted. That is approximately 1/20th of the card fee for that 5 pound transaction you gave. I think it’s easy to treat a factor of 20x as just another number. But at a certain point quantity is its own quality: the difference between 6% and 0.3% is so large that the second one might as well be zero.