If you previously purchased Geotag photos Pro 1 and wish to upgrade to the Geotag Photos Pro 2, you are eligible for a $4 USD discount (the price of the original app).
To receive this discount, you need to have a geotagphotos.net account that you created in the Geotag Photos Pro 1 app. Log in with the same account to Geotag Photos Pro 2, and the discount should be applied automatically.