I prefer the single use crates you purchase.
You can purchase as many as it takes to get the rare you desire. No key needed, you get to keep the empty crate or sell it back for 5% of original cost.
Make the crate available the entire month so players have ample time to purchase as many as they desire/afford. Make the odds of the rare no more then
1 in 500 and the super rare no more then 1 in 2000.
Example 1 (Gold Crate):
Crate = 500 credits sell back = 25 credits
Purchase 500 crates gives you a 1-4 shot at ultra rare and 1-1 shot at super rare.
250,000 credits purchase price, sell back empties for 12,500 credits for total
cost of 237,500 credits.
Example 2 (Silver Crate):
Crate = 100 credits sell back = 5 credits
Purchase 500 crates gives you a 1-4 shot at super rare and 1-1 shot at rare.
50,000 credits purchase price, sell back empties for 2,500 credits for total
cost of 48,500 credits.