Do you have Score points or the "Oasis Coins" (displayed bellow the map). The Secret shop works with Oasis coins, you gain those per wave and they cash out every time a match is completed. Either if you win or lose. If you quit the match they will not cash.
Right now I opt for waves to finish as the game's "challenge" is tied to the shrinking after each wave. In the future, I would like to add difficulty options but I think I would still keep waves to a set number. The inspiration here is how Killing Floor does it.
Mercenary Hunt is just a bonus mode, is not supposed to be wave-based at all, and still needs a lot of work.