Costco HAs were locked at one time, and only Costco could adjust them. Now, any ReSound dealer has the ability to service Jabras, and I believe any Signia dealer has the ability to service Rextons. They’re unlikely to service HAs they didn’t sell, but they can provide service, if they choose to.
Every so often, someone posts that they can’t get to the audi who prescribed their HAs, and no other audi will provide service, even for a fee. My Jabras (Costco) were sold in Illinois and have been serviced in Massachusetts and Nova Scotia (where the HA center replaced a receiver under warranty)…
If you buy from a private clinic, you’re likely to have trouble getting service for your HAs if you move or if the practice closes or if you need help while travelling, If you buy from Costco, you can get service as long as you can get to a Costco, at least in the US and Canada.
IOW, whatever HA one buys, fitting is either done by the seller or the user.