Adding a bridesmaid without making it seem like a pity ask?

I've been engaged since last March, and have had my BMs picked since before then. Just my 2 sisters and best friend. Over the past year, I've become closer with another friend, and have thought about asking her, but just kept putting it off. She's getting married 3 months after me next year, so last week I asked her to grab coffee and do some planning together. It crossed my mind that maybe it was weird to ask her to help me brainstorm wedding stuff when she wasn't a BM, but I didn't worry too much about it. Just thought it would be nice to chat about our plans together.
Anyway, big mistake. She walked in with a box, and my oblivious self thought nothing of it. So at some point bridesmaid dresses comes up, and I'm asking her about her scheme etc. and she says "hold on" and hands me the box. Crap. I said yes, obviously, and I'm thrilled that she asked me. But now I'm kicking myself for not asking her first, because I've been engaged longer and had plenty of time. I really want her to be by my side at our wedding, but I don't want her to feel like I'm just returning the ask, especially since she knows I already picked my other 3 (to be fair, I didn't really "pick" them, they were just the obvious choice). Obviously that coffee date would have been the perfect time to ask her, and I messed up. But how do I ask now, and make sure she knows it's not just reciprocal?