Can I get a receipt after paying a fine in Spain?

There are two ways to get a receipt,

• Online: you can request it at the end of the payment operation, introducing your e-mail in the given field. As the payment is done via credit or debit card, you can also ask your bank for the operation details. 
• Transfer: if you pay by transfer you will have to quote the reference number (top-right corner), and your bank will issue a receipt. [Subject to surcharge]

According to the Spanish law if you pay by card the procedure against you MUST be closed within 72 hours, so my advice is to call their information number 72 hours after the payment to confirm that the procedure is closed. If that is the case you can forget about the fine.