Have I ever forgotten to put stamps on a letter? Sure, I forget all sorts of things. I forget to sign checks, sometimes I forget and pay bills twice, I forget appointments, I forget metting people within hours of meeting them, I forget where I parked my car, I forget where I put my keys, I forget the names of my sister's kids, I forget what I had for breakfast this morning, I forget plots of novels and movies and TV shows. Basically, as the man said, I would forget my head if it wasnt screwed on.

Back on topic, if you're lucky they will send it "postage due". If you're even luckier the postman will have seen your note and gone to the trouble of correcting your mistake.

You're lucky in that you know this happened, so you can check to see if the letter arrived or not, and deal with it from there.