Mhh ... I move all confirmation emails from online purchases in a separate folder on my IMAP. So, I always have a list of all purchases I've made, ordered by date. I don't buy that much stuff online so keeping track using that list usually works fine. Except for xmas, I rarely have more than, say, 2 or 3 packages coming my way at the same time.

I simply keep checking that folder once in a while to see if I received what I ordered.