Your best bet is a Google Play Private Channel.
Its basically creating a private channel for your domain, for which only people part of your domain could access the app. There's a setting in your google apps control panel, "Allow users to access Google Play Private Channel" which you would have to untick.
Then when deploying the app check "Make this application available only to users of my Google Apps domain".
Have a look at this link, it gives you an in depth guide how to go about publishing an app to a private channel.
http://www.techrepublic.com/blog/google-in-the-enterprise/deploy-private-android-apps-on-google-play/
Alternatively, you could provide a download link to the actual apk file on your website (once your consumer has paid for the product) and just give them instructions on how to install the it. eg. Enable "Unknown Sources" on Android Settings, then install the apk file.
I hope this helps.