Day light *******

I saw and ad and so my son wanted sneakers so I sent WhatsApp asking if they are available and how much. They responded to say is R1700+ R100 delivery meaning I must pay R1800. I did that however I paid R100 extra by mistake meaning I deposited R1900 instead of R1800. I sent proof of payment they requested for my address and I informed them that I mean extra R100 how will I get it back. They responded by they don't do refund I will have to top-up next time for something, then I said they will not be a next time I want my money and they said in that case there is nothing they can do.

I have been ****** which is not fair. I really want my money back I am working very hard for it, for someone just to take it just like that.