The "-ess" ending is only used with choice words, manager not being one of them.

Some that I can think of are:

Priest(ess)
Sorcer(+er for males/+ess for females)
Wait(+er for males/+ess for females)
Act(+or for males/+ress for females)
Host(ess)

This one is AFAIK an AE/BE difference.

The word doesn't even exist in AE, or if it ever did, it's long since obsolete.

My impression is that it's rather dated even in BE, and that if it's still used at all, it's only in the very limited contexts you mention: a small, old-fashioned, family-style service business, like a local shop.