Spirit of Christmas?

I was at the post office. I had one piece of mail to send off. I walked up to the slot and there was a lady there. She had a stack of mail. I estimated that she would have to divide the stack into four smaller ones to fit them through the slot. Thanks to 911, they no longer have the door that allows you to make a big dump; all mail has to go through the slot. She was directly in front of the slot and I was next to it. So after he made her first drop and was "reloading," I stepped forward saying "Excuse me," and slid in my one piece of mail. Then I started to walk away.

"Or you could wait until I finished," she said.
I thought she was joking. I attempted a weak smile and kept walking.
"It would be in the spirit of Christmas, you know" she continued.
Since I am a little dense, I was just then starting to realize as I kept walking that she wasn't joking.

I had no idea that my little intrusion was offensive. I'm used to being around folks who would gladly give you the last piece of pie, or let you go ahead in line because you had just a few groceries, or step aside from the copy machine because you needed just one copy and they had a book to do. It never occurred to me that anyone would object to my stepping in for a split second; no idea that I was violating the spirit of Christmas.

Help me out folks, would you agree that I should have waited until she finished? Or should I have asked if I can jump in?