It pays to learn how to do it yourself.

Changing seals is something you need to do in regular intervals.
You could have a repair person check the back every time a seal is replaced, but that is both not necessary and far too expensive.

You will notice when a back needs seeing to. As long as there is no problem, having a repair person check it serves very little (none at all, i'd say) purpose.
Provided that the back is in good shape to begin with. Else you might want to have it serviced (have the only part that really wears - though slowly- replaced: a nylon stop. Once replaced, good for many decades of use) and then you do not have to have it looked at until you begin to notice something might not be quite right.

So find a good supply of seals (i make my own from a very cheaply available, but perfectly good foam), remember that you only need to change the mylar foil part if it is damaged, and just do it yourself.
It's easy enough, and doesn't take long.