Sorry, I misspoke: the API key is only for map views, not for geocoding requests.
Geocoding is limited to about 2,500, by IP address. The only exception is there’s a higher limit (to 100,000, I think) for Google Maps for Business accounts – but it’s very expensive.
You can also configure MapPress to use the nominatim geocoder, but in my experience the resulting geocoding quality is poor.