I feel that there is somewhat of a disconnect between your desire and reality. The comic shop's register is going to be nothing like the registers in a franchised retail or fast food, so if your end goal is to be proficient on a register, you might as well learn on the register where your job is going to be. The register they will be using in the CS will be old fashioned compared to the touch screen versions in the 'real world.' It is not using the register that is the skill, it is understanding the business, whether it be comics or Big Macs, that is the real skill you should be looking to acquire.