The price is frozen at the sale
When a patient buys a course, the treatment, the session count and the price are copied onto their purchase. From that moment the purchase stands on its own.
So you can raise your laser prices in March, add a session to the package you sell from now on, or retire it completely, and everybody who already bought one keeps exactly what they paid for. Nobody has to remember which version of the offer a patient bought, because the record remembers.