If you make it automatic, remember to turn it off if you have a buyer who wants to take a few days to buy other items and combine them.

If you're likely to be away yourself for a day or two you'll also need to remember to turn it off. I once bought something and the seller told me they had more of the same item that they'd list for me, and that they'd combine postage on them all - but then they went away over a long weekend without telling me and the unpaid process kicked in on the first item, which I found quite offensive.

I would have put up with it if she hadn't added insult to injury by lying about it and saying ebay had set it up and she couldn't do anything about it. After that I wouldn't have bought anything from her, no matter what it was, so I just wore the non-payment strike (my first and only one).