Forced to take more products in order to pay with my card

I went to buy a SIM card to port my number from my current network provider to MTN and was told it would cost R4 for the sim so I took out my card to pay as I had no cash on me but was told that I can not swipe my card for only R4, and was told I should buy something else from the store as the minimum they could swipe my card for is R30. This was something that I feel is unfair to a consumer with a limited budget.