If that is the payment agreement you have with her, then this method has been often recommended on this site: Give them the list and let them do all of the legwork, use THEIR gas and their time. But ......!
if you go that route, be sure you emphasize "NO SUBSTITUTIONS" on the list. Give them the name of the store where they will find things and NO SUBSTITUTIONS. If they walk in with, using the example above, "I couldn't find baking powder but here's baking soda......." Just hand her the soda and say, "Well, here your soda and when you bring me the b.powder, I'll be able to do your cake. Until then ........?"
But if you said nothing to her about "you buy the stuff and I'll make it", then that needs cleared up right away and up front.