Awww...don't worry about it-- it's done.
Yes, it is a little bit tacky, but I am sure most people will assume it was not your idea.
At my bridal shower, my mom presented me with a basket full of pre- addressed thank you cards during the gift opening. She had filled out the envelopes herself, so it was a little bit different, but I was mortified.
I actually flipped out at her for it, and she said she didn't realize it was weird to do, and said she did it to "make my life easier." Then of course I felt bad for flipping.
My advice is to let it go.