The bill can be paid at any branch of one three banks. I walked over to the World Trade Center where all three of those banks have offices. First I went to Banco Santander. I explained the situation to the teller, and asked if I could pay extra. "No, you can't," he said. I was going to pay the 50 pesos, but his system would not recognize the reference number to credit my apartment. So, I took back my 50 pesos, and walked over to the second bank, Bancomer. As you enter the door you take a number. I walked in to find that there were at least 30 people ahead of me. I waited for a few minutes and thought, "The heck with this." I went to the third bank, Banamex, and I did not have to wait long. Again I asked if I could pay extra. The teller called a manager over, and he said that I needed to call the gas company to see if I could do that. So I paid the fifty pesos. (There was no problem with the reference number.)
I decided that rather than calling the gas company, I would go to the office and talk to someone in person. Their office is in the neighborhood of Roma Norte... not too far away, and easily reached by Metrobus and a short walk. The lady I talked to at first was suggesting complicated solutions such as having the gas turned off when I am gone and turned back on when I return. I finally repeated that I simply wanted to pay extra at the bank so that next month's bill will be more than covered. "Oh, yes," she said. "You can do that." So that problem is solved. Next week when I return to the apartment I will go to Banamex again and pay enough so that I do not have to worry about the next bill.
There was one silver lining to all of this rigmarole. The gas office was only a couple blocks away from my favorite restaurant in the city, "Angelopolitano". I walked over there and had a delicious afternoon dinner of cream of poblano soup and pork loin with red "pipián", a sauce made with pumpkin seeds. As always, the food was superb!