Hi!
So today I implemented IAP using Unitys solution in my already published game. The IAP is working fine, but I can’t update my older version of my game, which is already on Google Play, with the new apk file. It gives me the error:
Failure [INSTALL_FAILED_UPDATE_INCOMPATIBLE: Package …signatures do not match the previously installed version; ignoring!]
I understand that it claims I’m using a other key to sign the package, but I’m 100% sure it’s the same file as I’ve used previously… Any help on this? 
Wait, is it because I have App Signing on in Google Play? So the key I’m signing with in Unity doesn’t match the one on Google Play store?
Yes. If you can’t update the app installed from the editor with the one published to the play store then that’s normal and expected if you allow Play store to sign your applications. The apk built with Unity should be signed with an upload key, but the app in the play store should be signed with a same key with which your previous versions were signed. This way your users who have had installed previous versions of your application will be able to upgrade to a new version without any issues.