You have actually made a legally binding contract with Paypal, otherwise they would never have let you use the account as a means of money transfer.
I know it is after the horse has bolted, but whenever I have had money paid into my Paypal account, I don't touch it until everything is clear cut and I know there will be no comebacks.
If you don't allow Paypal to take the money out of the account you said was guaranteed, I am sure they will take legal action to recover from you what would now be classed as a debt, and all the legal fees associated with it (those are the killer bits, a couple of hundred squid usually).
Paypal will be like any other credit agency, don't pay them, and you can be blacklisted for years. As soon as they start action against you, even if you then make a payment to them, you will already be listed as a bad payer, and it is a pigs own of a job to get off it. If you want to move house and apply for a mortgage, want a bank loan, a credit card, buy anything on tick, you may as well kiss it goodbye. Everything you want to buy will be a cash only transaction. If you already have credit cards and such, or bank loans, if they get wind of it, they might start putting the clamps on as well. Refusing to give any further credit or extensions.
I know all this because I went thru it many years ago, when I was made redundant in the early 80's, and I am still going thru it now, with my daughter, and even though I have offered to clear her small debts, she will still be blacklisted. I think it is either 5 or 6 years it stays against your name, so whenever anyone does a financial search about you during those years, you show up as a bad payer.
It is up to you to decide whether you want to live with a bad credit rating for many years for such a small sum.
There is no way you should blame Paypal, they are only protecting themselves, like any other financial institution would, and cannot be held reponsible for other peoples actions. As they say, buyer beware, in your case, seller beware.
If I was you, cough up, and see if you have any redress against the postal service, but I doubt it by what you have already told us.
Bogs