I order items online a lot, and if I choose to tell them to bill me later instead of paying right then with my credit card, I always get a delivery note taped to the outside of the package when it arrives. I have to remember not to throw this away, because it contains information on how to pay for the items.
Sometimes, I have to mail part of the note back in with my payment. This is usually the part with the order number on it so that they can verify my identity.
However, if the note gives me an option to pay online, I take it. This saves me money on stamps and envelopes.