How do orthos get paid for/by Invisalign?

Purely curious question here. I paid in advance for my entire treatment, and for every 2-3 months visit for 1.5 years already and extra braces (3 of them already) there's no extra charge. I spent 20-30 minutes every visit which is surely a billable time for other types of appointments. Yet they do not seem in a hurry to finish it and seem happy to work on my teeth until it's fully done.