Paying bills in Mexico is quite a bit different than in the States. In the US you don’t usually want to let your phone bill or cable go unpaid because it will be cut off and you’ll end up paying a large fee to get it turned back on. In Mexico, the postal system isn’t very reliable, so many places don’t even send out a paper copy of the bill.
Many times the way companies let you know that your bill is due is by cutting off your services. With phones this means you are no longer able to place out-going calls, until you pay up.
It seems to be very uncommon to pay bills through the mail, so most bills are paid by going to the place of business or the bank. There are some types of bills that can be paid at certain convenience stores as well.