Auctioneers impose a credit card surcharge.
Aldi and Lidl don't take credit cards.
There are lots more examples.
The guy has every right to impose a surcharge, but he is obliged to tell you about it upfront. He didn't so he waived the charge.
You have now got the original contract you signed up to. You have a legal and moral obligation to complete the contract.
He should not have been rude to you. But being rude is not illegal. Put up with it. Buy the product and enjoy it. Tell your friends not to deal with this guy anymore.
Brendan