What is the best way to bind software for android to a specific order?

Question

Android

I have a bike rental. There is accounting software on a PC that counts everything every minute, maintains a customer database, displays time, totals, discounts, etc. The whole thing is synchronized and stored on the server.
Now I want to implement an android client for my clients. Those. so that a person can rent it, install the software and be able to see everything the same on their smartphone. But I can't think of a better way to implement binding to his order. If there is one person, then you can link the order by phone number ... But in 90% of cases, 2, 5, 7, etc. are issued per client. bicycles. I would like everyone to be able to track the rental time in their smartphone. Maybe someone with more experience can give some ideas.
I would be very grateful to you.