Very cool crate, makes me wish I wasn't poor. Also here is an idea for shiny keys for mystery crates:
A shiny would be very expensive, and would give either a mythical reward from one of the crates featured in the mythical crate (example: winter axe), or the reward for opening one of those crates 40 times. If you think this would be too good, either up the price (I think 30c is a decent price), or make it not give the cosmetic part of opening a crate 40 times. It also couldn't give the reward for opening the mystery crate 40 times, but it could give one of the mystery crate mythic items. Hope you take this into account and maybe implement it, thanks!