This website:
What can businesses expect to pay for credit card processing? Find out the potential cost of credit card merchant fees in this guide.
gocardless.com
Indicates that card fees are between 0.25% and 0.9%, plus per-transaction fees that are 6-10p + 1-6p. So a £3 card payment is likely to incur something like 12p in fees. That’s 4%, which is quite a high proportional cost, but I’m not sure it’s worth saving the occasional 12p for the sake of pissing off customers whose lifetime value to you will be much higher. And it’s not like handling cash is free either, for a business.