Some banks will let you link a savings account to your checking account. Only keep $5 in it to keep it open. You can get a "debit only" card to use with the savings account.
You can then transfer (online) however much your online purchase is, from checking to this savings account.
If someone does try to use this card #, there isn't any money in the account, so the debit transaction will not go through.
It is VERY important that it be a DEBIT Card only. A debit/credit card may go through, and you could end up having to go through canceling the charge and getting the NSF fees waived, like in a credit card transaction.