OT: Back in the old days when all travel agents were brick and mortar and all bank account debit transactions were done using slips of paper (called checks) your payment would be "pending" for a few days (until the check cleared), There was no technology in those days for the bank to show you your current balance on a screen let alone an entry for the airplane ticket you just bought with "pending" next to it. So it was easy to write more checks than your balance permitted and have some bounce.
Paypal has a two tiered system for managing transactions. When you make a payment, the money comes first from a "kitty" in your name that holds funds from eBay sales, etc. If this is not sufficient, then your bank account or credit card account linked to your Paypal account is tapped. Paypal likes it when funds are transferred from one kitty to another during eBay transactions etc. since Paypal is actually holding the funds all along and takes a commission for every transfer made. Whereas Paypal, like any merchant, has to pay a commission every time new funds have to be infused into its system from a customer's credit card account.
