The GUI POS have some bugs:
1- If you work in Spanish language, when you tried to do the payment the system fails, (it show some values Grand Total and Balance not in Locale format) in English version it works fine.
2- If you have the authority to change prices it does not works, if you change the price (in the below line) it changes the item quantity.
3- When you tried to delete an order the message Delete Order is not translated (if you are working in another language)
Attached is the patch file with the changes for fix these problems and a message that must be added to the system messages.
Ubuntu 14.04 64 bits openJDK Postgresql