The unit is the size and the side. A support in medium left is not interchangeable with medium right, and a stock system that counts products rather than variants sends somebody home with the wrong thing.
Hire is a second inventory with a return date. What is out, with whom, since when and in what condition determines whether the shop still owns its equipment at the end of the year.
Custom work is a job, not a sale. A measurement, a supplier, a lead time and a fitting appointment when it arrives: without tracking, the customer rings before the shop knows the item has been delivered.
Reimbursement has its own trail. Prescription details, claim submission and payments arriving later behave nothing like a counter sale, and unclaimed items are money the shop has already spent.