It's just network policies that prohibit charging before shipment, so it wouldn't be problematic if this was an ACH payment.
If it was a credit card payment, most likely PayPal had just placed an authorization and was holding off on capturing. That would explain their aggressive timing -- if they let the auth hang for too long that would result in higher interchange fees, and eventually the auth would expire.
Doesn't PayPal get around that by charging your credit card to buy credit on Paypal, which is then used to purchase the item you want? Or do they only claim that when it is convenient to do so.
If it was a credit card payment, most likely PayPal had just placed an authorization and was holding off on capturing. That would explain their aggressive timing -- if they let the auth hang for too long that would result in higher interchange fees, and eventually the auth would expire.