Currently the container and GUI each do their own calculations and handle things like which enchantments are being applied, and how much the enchantment costs. Once the player is ready to enchant, a packet is sent to the server with the client data, and that data is verified.

This approach works, but it means twice as much code, and is over all much less efficient. The system should be rewritten so the client sends updates based on user input to the server, and the server send updates back. This should minimize potential for abusing the system, and greatly reduce the amount of code needed.