Tomorrow Stamps are a currency in Little Inferno.
The player gets Tomorrow Stamps by making combos, but it can also be dropped randomly by destroying an item. Tomorrow Stamps can be used to skip shipping, although it costs more stamps if the time is longer. Every 30 seconds remaining on an item is one stamp.
Spoilers: When you unlock the End Combo, the Tomorrow Stamp Indicator will be destroyed like the Coin Indicator.