...the amount I can withdraw is exactly $45 less than the amount sent.
The more important factor would be how much is your available balance in your Paypal account... is that $45 more than you can withdraw or the same? Either way, you want to be looking at your account activity.... charges, fees, reversals, refunds etc... whether the Pond5 payment was sent in two parts and one is still pending etc. I'm guessing you'll find some clues there and it's not just a case of having $0, Pond5 sending you $100 (for example) and you can then only withdraw $55 of it.