I really think you should try to make amends so you're on a courteous level. It's obvious you're never going to like each other, but it's never nice knowing that you're unwanted around your partner's family. You've said you never ring her except in an emergency, but I'd suggest maybe you could ring her this time and say something friendly about it being the most efficient way to sort things, and make sure your boyfriend has a great birthday? What does your boyfriend want to do? If not, maybe you could email her - say you just wanted to clear up what's happening. Be polite but not sugary-sweet.
This is just my advice - of course everyone's different, and I've never really had the misfortune to meet someone who is obviously such an unattractive personality. My boyfriend's parents are very different to mine, and I sometimes find it difficult in their company, but it's never their fault, just me trying to adjust to them. This is why I'd say that feeling awkward around your partner's family is one of the worst things ever! If it was anyone but your boyfriend's mother, I'd say stop bothering... ha!