Sellers can file the Unpaid Item Dispute 48 hours after the listing closed. Once filed, the buyer has a 4-day grace period to pay. If he doesn't, the seller can close the dispute, giving the buyer a non-payment strike. Two strikes will get buyers on most seller's blocked list.
When in a bind like this, contact the seller about the mail hold-up. He may be understanding and allow you a short extension on your payment.
Good luck!
Graciously Grin When You Growl