I still think it's dumb that if they're sending out multiple packages with codes to the same person that only one code from the current batch can be used. It seems like the more sensible thing would be to email out a coupon per month that's account specific if a customer is buying regularly or something. That way they could save on not having to print the stickers and it'd be easier to keep track of who's getting codes so that it doesn't feel like a tease when they're sending out multiples.
They probably just have them stuck on the box before they're mailed out. The person putting them on the box probably has no idea who already got one since packages are mailed out separately. And they figure the extra codes would just be given away. No loss to them.