I am tending to find that people whose credit cards have expired and who have not updated their PayPal account with their new credit card details will send Echeque payments - basically there is no source of "back up funding" - sometimes the buyers contact you saying that the funds have left their account and that you must now ship the item - however, until you receive confirmation from PayPal that the transaction is complete and the funds are in your account, you shouldn't send the item.
On one occasion, a buyer wanted the item ASAP and didn't realise that their credit card had not been updated on PayPal - we refunded them the original payment (Echeque) and then they instantly made a new payment with the credit card on file and we shipped the item the same day.