IDEA: Preview upgrades in store.
It would be nice if it was possible to test the upgrades in the store, without purchasing them, the purchase is finalized when exiting the store.
If you have equipped upgrades you can't afford, then the doors are closed until you un-equip it.
(Maybe even with a mechanic that makes it possible to steal upgrades you can't afford, like the next room goes directly to the boss battle & the boss is +10% to +50% harder, or something).