Do you actually log into P&P, find the prize, and buy it from there? Or do you just click the "Get it for ### points!" link from the postings here?
I noticed the Tasty Treats prize was gone from P&P very quickly. I think they normally do pull the prize descriptions automatically as they sell out, but they don't (or can't) remove the item ID from their database. So when you click the purchase link on the postings here -- postings that may have since been taken down from P&P itself -- it still adds the prize to your cart.
In the future I'll make sure to remove the "Get it..." link from any new prizes that I copy and paste here, and I suggest everyone else who posts prizes do the same.