Showmax payment

I recently downgraded my showmax plan which costs 255 and that's the amount that was taken of my account. So the next day after the payment of 255 went off my account I upgraded my account to showmax pro which costs 349 so they asks for payment which I made via my card thinking they will take off the difference of 255 from 349 but they did not they took of the entire amount of 349. Now when I query they say they can not refund me the 255 will he converted into a voucher so when I downgrade I can use it. But I am not going to downgrade but I just get told they can't not refund me . Or take it for next month payment and I pay the difference.