I'm very sorry that your aunt was unappreciative of your efforts on her behalf.
I don't think you handled it as well as you could have done. I think it would have been better to give her 24 hours to get over her clearly terrible mood and then contact her. It's not what you said that was wrong but the short time frame and the single medium and the capacity for escalation were all against you:
- Text message is not the best way of communicating urgent information
- Text message is a bad medium for emotional communication because tone and nuance are stripped out
- Addressing the issue with your aunt so quickly didn't give her time to think over what she'd done and realise herself that she was in the wrong
- going from close relation to cut direct in 24 hours doesn't give you the time you need to decide if this is the best way to handle the situation
So, when you encounter aunt in future, the best thing to do is to keep certain key concepts in mind. Recall the true facts of the matter and the facts you wish to convey. eg Your message that the med could not be filled did not reach her, she was abusive to you, you don't wish anything similar to occur in the future. Those are the key facts. Then you can cut your cloth to suit her actions.
Perhaps she will apologise in a heartful way - then you can choose whether or not to accept her apology and whether you would ever fill meds for her again
Perhaps she will not speak to you, in which case you can need to do nothing and if others ask about the disagreement say "that's a private matter".
Perhaps she will speak but not apologise. This is the worst case scenario because the true cut direct would require you to blank her. I think at a family party this would be rude and potentially cause a lot of drama. Instead I would advise replying briefly and politely. And if she raises the issue of the meds say "I'd prefer not to discuss that" or "I'd prefer to discuss that some other time / in private".
Keep cool and keep your aims in view.