A double-stamp day is a short promotion in which every visit or item that normally earns one stamp earns two. It might run all day, or for a few hours such as a quiet afternoon. Customers like it because it brings their reward closer without needing to understand anything new. Shops use it to pull visits into a time that would otherwise be slow, to give a reason to join on the spot, or to mark an occasion such as an anniversary.
The cost is not the extra stamp itself, which is only a fraction of a reward. It is that the reward arrives sooner for everyone who visits during the window, including the regulars who would have come anyway. A double-stamp day pays when it brings in enough extra visits to cover the rewards it brings forward.
In practice
A cafe has a ten-stamp card for a free coffee that costs $1.50 to make, and a margin of about $3.80 on each coffee sold. On a double-stamp Tuesday afternoon, 120 member coffees are stamped twice. That is 120 extra stamps, worth 12 free coffees once the cards fill, or $18 in cost to the cafe.
To pay for itself, the afternoon needs at least five extra coffees that would not otherwise have been sold: five times $3.80 is $19. A quiet afternoon that normally sells 40 coffees and now sells 75 clears that bar easily. A Saturday morning that sells 300 coffees either way does not clear it at all; it just hands out rewards earlier to people who were coming anyway.
Run them rarely, in your quietest window, and tell staff exactly what counts. A double-stamp day every week becomes the day regulars save their visits for. For the full sums, see double-stamp days and when they pay.
In Counter Club
There is no double-stamp setting. A double stamp is two presses of Stamp, and the rule for when to press twice is yours to set and write on the sign. On a paid plan, a campaign can tell the club about it. See what counts as a stamp.