Game developers manipulate player desires by presenting the same options differently. Player reactions are empiricably testable with cash shop setups.
I frequently cite the example of having a “hunger” debuff versus a “well-fed” buff. These can be designed to be numerically identical, where the character has higher base stats that are debuffed by hunger or lower base stats that are buffed by food. You balance content around the higher number in either case. Players will complain about a hunger debuff but feel like they have been given something extra with a food buff. Even if the numbers are identical, humans are unhappy if you tell them you are taking something away from them, whereas they barely notice if they fail to gain something.
Many cash shops have some sort of lottery option. You can give the developers $X for a chance at items or whatever. What you see at least as often these days, because we would predict that it works better, is giving you a lottery ticket or prize you can pay $X to unlock. In the former case, you can play the lottery by giving me $X; in the latter, this lottery ticket is now yours, but you cannot redeem it unless you give me $X. Same lottery, same prize, same $X. If you doubt which implementation yields more sales, look at where the developers are betting. Team Fortress 2? Locked crates with keys in the cash shop. Guild Wars 2? Black Lion chests with keys in the cash shop.
Developers can make this more concrete by adding time pressure: the box/ticket expires in a week or after the event. Some players might still see a locked chest or lottery ticket as a failure to gain, but if it is going to disappear in a few days, they have definitely lost something, even if only an opportunity. The perception of scarcity also plays in here; you always have access to thousands of TF2 crates and GW2 chests for a few cents, so it is harder to instill the idea that you are losing any opportunities, while other games might make those drops less common (but still give the player frequent opportunities to buy things). Hence TF2′s time-limited crates, and doesn’t GW2 have occasional seasonal Black Lion chest items?