Feb 2019 - Now with 1.13+ support! (tested on Spigot v1.13.2)

This enables enderpearls to store experience, to safeguard XP from deaths and to allow trading XP like physical items.

Left clicking with an enderpearl in hand deposits up to 16 (configurable) levels worth of experience into the pearl, making it look like an Eye of Ender
Left clicking with an imbued Eye restores the experience and returns a plain pearl
Experience lost/gained still takes level scaling into account (from wiki's values)
- For example, with the cap set at 10, up to 160 xp can be stored
- If at level 16, storing 160xp (the xp to get from level 0->10) will take you to 11.4
- If at level 10, storing 160xp will take you to 0
- If at level 20, restoring 160xp will take you to 22.4
Right clicking imbued Eyes tells you how much XP is in them and won't throw them, as that'd waste the XP
Imbued pearls (Eyes of Ender with a custom name + lore) can be dropped on the ground and left in chests, including through server restarts
/pearlxp <level> imbues the held pearl with a specific number of levels

UltiByte made a video of v1.3.4! (missing the new custom imbued pearl name/description)

maxLevel - The max experience that can be stored in a single pearl (so maxlevel 16 would be the experience from level 0->16, or 352xp)
emptyMaterial - The item material to use for the "empty pearl".
imbuedMaterial - The item material to use for the imbued pearl, can be the same as the empty pearl
expReturnRate - A multiplier for how much experience to return to the player on restore. 0.85 would impose a 15% "penalty" or "tax"
alwaysBlockRightClick - Default off, entirely disables enderpearl/eye teleportation (not just on enchanted stuff)
Customizable Messages

The source code is in the jar.

PearlXP