Find out how much you have to pay by going to the website noted in other recent posts, then you know how much to send.
It really isn't hard to pay your taxes yourself - go to the municipal building, if someone asks why you are there say 'predial' and they will direct you where to go.
Not any harder that paying for your vehicle registration at the Recaudadora on Degollado; take your circulation card, wait your turn, hand over your card (you get it back), pay your money, get your receipt, away you go.