Entiendo que en este sentido, "rain check" es un vale o un cupón que una tienda puede darte para próximas compras o descuentos.
¡Espero que los enlaces te sirvan!

Es un "idiom": "Rain Check" es una promesa aceptada para recibir algo, pero en el el futuro.

"I'll take a rain check (American, British & Australian informal) also I'll get a rain check (American informal)
something that you say when you cannot accept someone's invitation to do something but you would like to do it another time (often + on )
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rain check (on something):

a reissuance of an invitation at a later date. (Said to someone who has invited you to something that you cannot attend now, but would like to attend at a later time. *Typically: get ~; have ~; take ~; give someone ~.)

http://idioms.thefreedictionary.com/I'll take a rain ch...

This means: once the reduced price items are all sold out, the offer expires, even if the items are restocked at a later time.

It is not a literal translation, but it is the most common way of phrasing it in Mexico.