Instruction provided by @invitu
1- Install Cups on the posbox:
- The posbox is a read-only system, you have to chroot it to install cups
- Run a posbox on a raspberry-pi and plug a pen drive containing another posbox image
- chroot the posbox in the pen drive
- install cups with apt-get
- plug the epson on the posbox
- setup a Raw queue using usb://EPSON/TM-T20II in cups and share it (remove the "?serial=xxxxxxx" part if you want to use the image with another printer
- there is no driver for epson printer for raspbian, that's the reason we must use raw queue