With my invoice, I always let them know to contact me within 3 days (per ebay rules) and ask that they pay within 7 days.
I would contact the high bidder and check with them about how they intend to pay and when they intend to pay, in a nice way....
Do they have decent feedback?
Also, just as a piece of advice, you should put payment terms in your auction, and whatever other terms of service you wish to have....