After a bit more hunting, I also found the offices where I needed to pay for electricity and water. Also cash transactions (although I paid cash to an automated bill collection machine at the electric company). There may be an easier way for me to pay bills, but I haven't figured it out yet. To pay the rent for my apartment I put cash in an envelope and deliver it to my landlady's house. The only bill I know I can pay online is the cable bill -- it's good to know that the cable folks trust their own connections.
Perhaps I could pay some bills with checks, but I wouldn't trust the mail delivery system. I'm posting a photo of the mailbox where I receive my bills. One single, overstuffed, open mailbox for eight apartments. Yup, 8 families receive their mail in this box. Some bills stay in the box for weeks, I've noticed. Many people have bank statements gathering dust in this mailbox. I wonder if identity theft is a big problem here? In any case, since this little mailbox has lowered my confidence in the Mexican postal system, I will continue to pay bills in person and in cash. Now that I've figured out where to go, I may even pay on time!