Signing up briefly in order to receive a discount

Many stores offer a discount coupon to someone who joins their mailing list to receive updates, promotions, and advertisements. If I received a discount thanks to joining the list, may I unsubscribe from it immediately, or must I remain on it for a certain amount of time?
On the one hand — I received a discount for this, and it is not fair.
On the other hand — legally (and also practically...) I can unsubscribe whenever I want, and they cannot force me to continue receiving messages. Perhaps they gave the discount with the understanding that I might unsubscribe within a minute?

Shalom u'vrachah,

It is permitted to unsubscribe immediately, if they did not stipulate otherwise with you.

The seller also knows that you can unsubscribe immediately; rather, based on experience, there is a percentage of people who remain signed up, and that is worthwhile for them.

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