Mine isn't so much a pay it forward, as a "from the other side" story.
Years ago, we lived in a very small town in the mountains. A family we knew was having a very, very rough winter, and we knew Christmas was going to be pretty bleak for them. DH decided that he wanted to play Christmas Angel... we budgeted $150, gathered up two weeks of groceries, a turkey, and small presents for each of the four family members. The hardest part was getting it all to the front door without their dogs barking! (DH spent a week ahead of time dropping by at random moments and giving them jerky, so they figured he was a buddy).
Christmas morning, I had the little boy of the family in Sunday School, and we were talking about the gifts of the season. He was so excited to share about their Christmas morning, and the things that impressed him most were the supplies to make cookies, and that their Christmas Angel *must* have been sent by God, because only God would know that his Mom liked mint tea best, and there was a whole box of mint tea, and a new teacup.
Fast-forward almost a decade. We were living in a new state, and DH had been out of work for seven months. He was taking any small odd job he could for a bit of money, but even that was petering out. We were coming out of the grocery store with about $30 worth of stuff, which had to last about a week, and a small car pulled up in front of the door.
A smiling family of four inside rolled down the window and passed out a little white envelope. "Merry Christmas to your family, and God bless you!"
We were delighted to be given a card by strangers-- "Merry Christmas to you! God bless you, too!"
We talked about how good it felt to have a heartfelt, cheery wish given to us, all the way home.
When we unpacked the grocery bag, we sat down to read the card. Inside were tucked two $100 bills, with a small note about hoping it would be a blessing.
Well, yes. Yes, it was a blessing.
So we got to spend that Christmas knowing two things: how good and humbling it felt to be someone's Christmas Angel, and how good and humbling it felt to be the recipient of a Christmas Angel.
We've since had the chance to "deliver Christmas" again, and it's a tradition we really, really enjoy.