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 *******, the shop assistant said that this was ANTON OOSTHUIZEN's rule. Smells like tax ***** to me. ******* and discriminate.