I think it might be best for you to try to develop some strategies to compensate for your ADHD inattentiveness before telling your boss about it. Sometimes bosses will get scared off by a diagnosis like that, thinking you'll never improve your performance. Yes, it is illegal to discriminate but that's extremely hard to prove and they can always give a different and legal reason for firing.

Meanwhile, you can do things that will help keep you on track. For instance, write checklists and tape them to the desk/register. If you lose track you can refer back to the checklist easily. Prioritize your work tasks on a TO DO list and only try to do one thing at a time. When you're interrupted, take care of the customer but then immediately go back to your TO DOs.

Good luck!