Customer enters location / zip code --> Text field populates GPS coordinates
Coordinates could be in the 1. DMS system (Degrees-Minutes-Seconds) or 2. DD (Decimal Degrees) system
Ex: Customer inputs London, UK --> Text field would auto populate with either 1. 51°30'26.804" N, 0°7'39.955" W (DMS System) or 2. 51.5074, -0.1279 (DD System or alternatively 51.5074 N, 0.1279 W)
It looks like there are many APIs that could do this, and since you already have this API in use for creating pictures of custom maps using locations, maybe you could add on an API integration to produce coordinates in text form.
Many successful Etsy sellers producing popular star maps/GPS maps use coordinates to customize their creations.
Thanks!