"...for anything that Hobbits had no immediate use for, but were unwilling to throw away, they called a mathom. Their dwellings were apt to become rather crowded with mathoms, and many of the presents that passed from hand to hand were of that sort."
Fellowship, Prologue
After you have send the order in you get an automatic reply, giving the relevant payment details. After receiving your payment the parcel is mailed to you. Of that you get a confirmation via email as well. So the principle is "you pay, we mail".
