consumer charged extra when paying with debit card

I printed and laminated an A0 size poster. They charged me R570 when i wanted to pay with a debit card. And charged me R470 whhen i opted to pay cash. Hoeever, the tax invoice they gave me stated that I payed R570 (vat total of R70). When asked why they do this, as this is illegal, the shop assistant said that this was ANTON OOSTHUIZEN's rule. Smells like tax fraud to me. illegal and discriminate.