We used to have a system where you could buy XP for your user. While this was convenient, it was also not very good for the economy. It pulled XP from an unlimited source, which is always bad for any economy.
So psiber helped me to change the system. We now have a command to bottle your existing XP instead. You use the command /bottlexp and it will wipe the XP of your player (in increments of 10, so if you have 55 xp points, we take 50 points and leave the five) and instead give you bottles with that XP (5 bottles in this case) into your deposit. Please be aware that XP bottle use is by design (from Mojang, not us) not 100% accurate. While we will give you 1 bottle for each 10 XP that you have, using that bottle will give you anywhere between 3 to 11 XP.
This should encourage people to rather either conserve their excessive XP or sell it at the shop, hence it will be used in the system instead of buying from an unlimited source.