Just a small update and change for Peons or higher. The original method of storing XP has been removed (right clicking a lapis block) due to potential lag issues when attempting to cash out 200 levels of XP or higher.
This feature has been changed to /xpbottle instead. This method works similarly, but instead of hundreds or even thousands of XP bottles, you will get one (1) XP bottle with the amount of XP you would like to cash out.
In order to use this, figure out how much experience points you have (not levels) by typing /xp. Once you know how much you have, you can /xpbottle <amount> to save it in a bottle.
For example, 50 levels of XP is roughly 5345 experience points. If you would like to add all 50 levels into a bottle, simply type /xpbottle 5345. You may also do less, if you would rather have an even number such as /xpbottle 5000.