Obligatory sorry for formatting, I'm on mobile

So, to give context, I [21F] had a boyfriend [21] for around 3 years who was manipulative and mentally abusing before I broke it off ~9 months ago. He was nice to my parents, especially my mother, who had no idea how he was when we were alone. 

Flashforward to after the breakup (and many "suicide" attempts later, in my home while I'm at work), he's still keeping contact with my mom to brag about how great his life is and this and that.  My mom continues to bring him up after us being freshly out of our relationship, so I message him one day to not talk to my mom anymore and how it's extremely weird that he's still talking to her and to come get the rest of his things out of my home. He gets defensive and acts as though I'm being angry and questions me, since he had gotten a new job, "Are you jealous of my success?" So I block him.

And so he decided... to act like a small child and screenshot my messages and send them to my mom, whining that I didn't want him messaging her anymore. After a long conversation with my mom, she agreed not to talk to him anymore nor to bring him up. Months fly by with him having no contact with my mother, and all is well. I have a new boyfriend who's incredible, understanding and kind. My relationship with my mom is better than it's ever been and I love her to bits (after pushing her away for years because of my ex).

And now to today.

Obviously the first of January is a holiday for post, but I always check the mail out of habit since we get a lot of mail stolen, especially when no one else is home. Today, I found a letter with only my mom's name on the front, with no address or postage (which I believe is technically illegal). Now, she isn't home and I had a feeling about the handwriting. And I open it, and sure enough, it's my ex boyfriend sending a birthday card to my mom. It has some emotional throw-up inside and some cash. So, I took the card and some extra cardboard I had laying around and tossed it in our firepit and burned it. I made sure you couldn't tell I what I had burned, or even that I had burned something. Just ashes.

I really didn't want to give the card to my mom, but I felt horrible for keeping a birthday card away from her, even if it was from my ex. Should I have done something else...? Am I the asshole?