I think it depends a bit. I sell antiques. People sometime take a few days to pay. After three days, I usually send an invoice (although you might do that sooner to show the combined shipping rate. Could they be waiting for that?).
The another reminder at a week. At 10 days, I file an unpaid. That's probably loser than other sellers but not everyone lives in I gotta have tomorrow world of Amazon and can make a purchase and wait awhile.
