I've found out that not everyone mails payments right away, especially if they have to go out and buy a money order. Don't forget that we had a holiday weekend, which always slows things down.

I would send them an email saying that you've invoiced them, but still haven't heard back from them, on how they intend to pay, and when you can expect payment. Sometimes they just need their memory jogged! If you don't hear back in a couple of days, fine a non-paying bidder claim.
That usually gets their attention, FAST!