Hi One^Love,
Welcome

I'm sorry that a perfectly normal day turned into such a disaster. I know how frustrating that can be. It just leaves you confused and wondering "What just happened?"
Does you W often get violent when she gets angry? Is she still at home?
Being off medication can make a huge difference in someone's behavior. About a year ago I began to wean myself off of Effexor and for 2 weeks after I made a step down in dosage I became an emotional mess-sudden anger, yelling, crying, almost panic attack like emotions. I felt dizzy and sick to my stomach all the time. I got these little brain zaps that made me feel weird. I can imagine not having medication for more than 2 days is intensifying her already intense emotions even more.
She grabbed alcohol, some popcorn, and other snacks. I told her while checking out to put them at the back because we may not have enough. I walked away to get a cheaper can of biscuits and when I got back the total was $20 over what we had, which she knew. I told her that stuff had to be taken back, she got upset and asked for the keys. I said no, go wait outside if you want to get upset.
I think this is where things started to go wrong. Your W may
know that the spending could not go over budget, but was she able to
understand that? She had her mind set on getting some extra snacks. When she was told she couldn't get it, she may have felt rejected and shameful that she had overspent or that she had to put things back. She may have even felt like a child being punished. Understanding reality can at times be difficult for someone with BPD because for them feelings=facts. Us nons often try to deal with the facts, but in reality, working through the emotions the pwBPD is feeling first can help us get to dealing with facts. If we skip the feelings step, then it ends in blow ups.
Being more empathetic towards our pwBPD can go a long way. Try practicing here. Would you write out a way that you could tell her in a more empathetic way that the extra food is over budget, taking into account her feelings?