So you call her (because she won't do this if she calls you, unless she is extremely mad at you, simply because she doesn't want the liability of having started the fight).. so.. you call her, and talk a bit and she's like "Hey could you drive to St. Louisville and pick up these red shoes for me that I bought 3 days ago but forgot to take from the store?"

You're like. The fuck. I'm right here, working on my Favorite Thing (TM), deep in the zone and she wants what?

What works everytime for me is: "I'm busy right now" (this alone never works) and then "...what I'm doing is so much fun"

Score. She will immediately start thinking of fun things, and fun things are good feelz.

She doesn't even (have to) care about you having fun, but unless she is either completely rational, or not just shit-testing you, but also actually an asshole (both cases not very likely), she will stop short of actually denying you the fun.

My SO then mostly replies something like "Hmm.. well, OK, I guess I can do it later myself". Don't expect her to actually care about what you're doing (in a later conversation perhaps), but you've thrown her a good feelz bone which she is now happily gnawing on, and you're free to go.