Bill, I believe 'rain check' comes originally from outdoor sports -- baseball? -- where, if the game was called on account of rain, the attendees would be given a pass to for the day the game resumed. Could be a concert, too, any outside event that was rained out. The ticket that let you come back later is called 'a rain check'.
Over time, the ticket has come to be given for sale items that are out of stock when you go to buy them. If the store no longer has widgets for only $17.95, even tho the sale is supposed to run till Tuesday, I can ask the clerk for a rain check. She will write the name of the item & price on the slip of paper, and I can buy it at that price when the next shipment comes in three weeks later.
Blessings,
Barbara