If you enroll in the Pay-As-We-Play program, they don't invoice you. They charge your credit card as soon as a home playoff game is clinched.
So, when we clinch the division, within a few minutes they will charge your credit card for the first playoff game. As soon as they win in the divisional round *and* the winner of the other AFC game is a lower playoff seed they charge you within minutes for the AFC Championship game.
If you don't enroll in the program, they invoice and you front the money and if the games aren't played in Foxboro you get a credit on your next year's invoice.
All-in-all, I prefer Pay-As-We-Play. Zero chance that your seats end up on Ticketmaster because you didn't pay the invoice by the deadline and you don't have to front the money until it is assured that you will be attending a home game. Plus you can earn credit card points, so a double-dip.