The military prescription formularies have two standards - the drugs that the active duty folks can get and the drugs that the retirees can get. If you get a drug not on the formulary for your particular status (have to go downtown even if it’s on-hand for active duty)), they will leave you a higher co-pay.
I go through a base hospital and over the years there has been no difference in where and how I get my prescriptions and where they come from, from before my retirement to after. Going mail order is going to be different and many recruiters, reserve active duty staff not near a base will also have to go through other routes for care and prescriptions.
I’ve read through the formulary several times over the last 22 years and I haven’t seen any problems. Price fixing and gouging is another matter.