Welcome, Mrs. H.!
Erstwhile customer?

Or--take a look at the last line in this def. from Webster's 1913 dictionary:
Erst
Erst (?), adv. [Orig. superlative of ere; AS. rest. See Ere.] [Archaic]
1. First. Chaucer.

2. Previously; before; formerly; heretofore. Chaucer.

Tityrus, with whose style he had erst disclaimed all ambition to match his pastoral pipe. A. W. Ward.

At erst, at first; at the beginning. -- Now at erst, at this present time. Chaucer.
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First customer? Current customer?