Me: no thanks, I'm going there later to do some shopping myself
Aunt: But we could combine trips! It's wasteful to both drive there on the same day! The responsible thing to do---
Me: ok, I give up. Please get me cat litter and cat snacks.
Aunt: Ok!!!! :)) Which ones? And what isle? What color is the package---
Me: exasperated Are you serious? I caved and gave you what you wanted. Now you're asking for more information? I told you I wanted to do my own shopping, and you fought me on it. I'm making a peace offering by giving you two items to buy for me, and you're saying I need to go find the names and package colors and isle numbers? Please just be satisfied with what I gave you.
The thing that she wants is to feel good about buying me something. But I don't want that. That's the disagreement.
I imagine most people would see me as the asshole here: she can't buy the right item if I don't tell her what it is. But I clearly don't care about those details; if I wanted something specific I would have told her that. The only reason I mentioned them was to appease her. I'm giving her something she wanted, something I wanted for myself, and she's demanding more.
So your plan for satisfying your aunt's presumed intent to reduce waste is to create more waste?
That's still in the "jerk" zone. Could even be considered petty.
I don't get to act on presumed intent, I get to act on present behavior. We'd all be a mess if we were obligated to guess what everyone else wanted. And the behavior was, "I want to buy something for you" --> my agreement --> "that's not enough for me, give me more". Notice that waste never came into the discussion, even after the conversation ended with me cancelling the purchase of the products, which you could have intuited meant there would be no waste
Aunt: But we could combine trips! It’s wasteful to both drive there on the same day! The responsible thing to do—
Me: ok, I give up. Please get me cat litter and cat snacks.
Aunt: Ok!!! :)) Which ones? And what isle? What color is the package—
It's wasteful for both of you to drive there separately, therefore her intent isn't something to be guessed. It's laid out quite plainly in front of you.
You offered the cat products for her to purchase. It's not something she asked for specifically from the conversation you have shared here.
Not her cat. She doesn't know what brands your cat likes or what they look like. It's very reasonable to ask for this kind of details in case what she chooses to buy isn't agreeable to your cat.
However, you're fully in your right to choose not to socialize. You just did it wrong this time.
Your mistake was the first answer.
Me: no thanks, I’m going there later to do some shopping myself
You engaged and set the tone. What followed was a complete wreck. Honestly, you and your aunt are on different wavelengths in terms of social customs. Blame it on a generational thing if you don't get why and leave it at that.
All in all, you should have probably stopped with "no thanks." Everything said after had you make trouble for yourself without reason.