Is It Normal That my Ortho Charges Me for Retainers (600$) After my Envisalign Treatment?

Each doctor has his own fee schedule and different doctors may charge for  procedures differently.  In orthodontics, the norm is to have all expected expenses spelled out in a contract before treatment starts, so there are no surprises.  It sounds like you are surprised, so I would suggest re-reading the contract that you signed at the beginning of treatment.  Perhaps only a particular type of retainer was covered.  The contract should govern and simplify the situation.  And, of course, you should speak to your doctor to arrive at a mutually satisfactory solution.

This should have been spelled out in the initial contract you signed for your treatment....If it was not then I think you have a legitimate beef with the orthodontist if he/she wants to add an additional charge.

I always include the first set of retainers in my total Invisalign fee, and I explain that to my patients at the onset of treatment. I do charge for new retainers after the first set. I suggest changing the retainers every 3-4 months, as they tend to lose their rigidity. Invisalign has a system of Vivera retainers which will be shipped directly to you every 3 months.