Keep things simple. You arranged to meet her for sex, the interaction is over. Don't extend it needlessly (she should know this herself, it's part of discretion).
It's nice to be nice, but you can help an old lady across the street, or donate to a kids charity, you don't need to run errands for a prossie.
If she replies, don't reply back (especially if you're not going to book her again anyway).
Or, politely say something's come up and you won't be able to get it.
It's not a big deal, she can get it herself.