This happened just few minutes ago. It's just me(17TM) and my mom(51F) at home and we're going to my grandpas house because it's the birthday of the daughter of his neighbor and long time friend. Me and her used to play sometimes as kids but as I am older than her we grew apart and haven't really talked the last 4 years. We started to talk again a bit because her father got her a horse for her birthday last year (they have a farm) and because I am in high school rn and she was preparing for going herself and thus didn't have much time I volunteered to help out with the horse whenever I could. My mom and grandpa help out too. That being said, my grandpa, mom and the girls dad agreed to all come together and buy a new saddle for her birthday this year (I didn't know this until today). So imagine my surprise when we're preparing to go to the car and suddenly my mom asks me to sign a birthday card. I already plan on wishing her happy birthday when we're there so I asked why should I? She said she's putting the card in an envelope with the money she's giving for the new saddle. I told her that I don't want to sign under something I didn't even know about until today and it wouldn't be fair since she's giving the money and not me so I don't want my name on the card. I told her multiple times that I have a gift for the girl(a new halter and homemade cookies) and I will wish her happy birthday when we're there. She just got mad about how I'm making things difficult and to just sign it but I refused, again saying that it wouldn't be fair since the money isn't from me. So AITAH?