Actually, for Canadians and Americans with a PayPal (confirmed?) account, when you have a balance lower than your purchase amount, PayPal will automatically take money from either your bank account or your credit card. If it's from your credit card (you can set which method it uses) it's instant and treated like it was a PayPal balance from the sellers perspective: if it's from your bank account PayPal treats it as an e-check (e-cheque) and it has a 4-5 day hold for the seller.
