It will most likely come back but your postman might have taken care of it. Once, a while back, I brought a letter out to my mail box as the postman was arriving. I handed him the letter without realizing the postage had gone up. He said no problem and reached over to a huge roll of 1 cent stamps. He said a lot of them carry them for the first few days after postage goes up. It costs the post office less money to provide a couple 1 or 2 cent stamps than to process and return a letter because it is missing postage.
On a side note, you know you can use the interwebs to pay bills now, right? It should elevate these kinds of mess ups, at least for time sensitive stuff like bills.