Yeah I know and I'd feel bad which is why I'd like to somehow pay them back for their prepaid shipping label.
1. Prepaid shipping labels don't cost money. Label only gets charged when it's actually used to ship an item.
2. In my opinion this is more of an ethical question than a legal one. I doubt there are any legal ramifications for not sending it in, but you may want to ask yourself if the situation were reversed, and PFS promised to pay a profit, you would not appreciate them backing out of the deal. There were at least two coin deals this year that turned out to be way less valuable than anticipated, and the dealers kept to their agreements.
3. In the future, if you want to take a gamble, don't commit to selling it to a dealer. Buy it first, and if it turns out to not be as valuable as anticipated, ask some dealers if they are willing to take it and have any more slots.
4. Perhaps a good way to do this is to ask the dealer in question to meet you in the middle on price.