i think that sounds reasonable. it gives them a timeframe to respond and realize their mistake, while also still getting you your money back - possibly include your paypal email or name in the message so they have a better chance of finding the transaction? sorry to hear this happened to you, though, that's really bizarre!
I got an adoptable about a week ago, and the artist posted their paypal info and I paid that day. I hadn't heard anything back from them for several days this week (like note with the file of the separate of the critter), so I went to check in on deviantArt again and discovered that the artist had blocked me.
I messaged them at their paypal email to ask what was up, and if there were any problems with the transaction, and provided the transaction ID. I had also given my deviantArt username in the note for the paypal transaction. They responded saying that they block people who don't pay. I also saw that they had resold the adopt to someone else.
I'm willing to believe that it was just a careless mistake on their part, not paying attention to their paypal, and nothing malicious. I don't want to bring the adopt owner into any drama, and I don't care to accept another adopt or anything as compensation (just bad vibes, you know?). But I still need to get my money back, and I've not had to do this before, so I wanted to ask for outside perspective on my next step as a sanity check.
Bottom line, I paid for an item and the item was not delivered, and was in fact sold to someone else. If I say something like, "Please send a refund by this coming Monday or I will initiate a paypal dispute/back-charge", does that sound reasonable?
Yeah, I provided my PayPal email again (and also emailed them from that address), as well as the transaction ID from my PayPal receipt.
I asked for a refund and they readily agreed, but also asked me to initiate the charge-back, since they could still not find the transaction on their end. Hopefully this will go smoothly from here, especially since they agreed via email to the refund.
In a completley unrelated transaction, I paid a commission invoice today, and I notified that artist that I'd had a problem recently with PayPal. With luck, it was just a one-time fluke/careless error, and won't cause any issues for the other artist.