When somebody wins one of my auctions, I send a mail with the cost, but also add, that "if you'd like to buy more, you're welcome to do so and save costs on shipping. But if this is the case, please let me know that you haven't finished, so I'll know it's not just a case of you forgetting to pay"
I've had orders run up to almost a month and this can get a little difficult to keep track of. If I'd been more of a 'big seller', it would not be possible to hold items for that long - I've already messed up a couple of times...
A wins an auction, I check my ongoing auctions for current bids, and see that A is also the highest bidder on another auction, ending in a week. I send a mail saying "you'll get the bill once the other auction ends". Unfortunately A gets outbid on that other auction, so his name never shows up again... until I discover the first item, still unpaid for, and investigate... ouch, my bad!
As a consequence, I made a flimsy system using stick-it notes with the names of current bidders. If a name disappeared from that list, it meant that he'd been outbid and I'd go back in my files to find the bill. Very time consuming and not fail-proof at all, but at least it's a bit better than nothing at all.