I downloaded the apk for the app Contacts Optimizer and had it installed via sideload. I would like to upgrade the app to PRO. This is done using an in-app payment. Trying to use this tells me that "Cannot initialize connection to in-app payment server, please try again later." I guess the reason is, because I have no Google App store on my phone. How can I use in-app payment without Google Play?
I don't think that Google billing is possible without the Google apps installed.
However, there are some apps that offer in-app purchase emulation. In most cases, they work and in addition, you don't have to pay. Lucky Patcher is an app which does in-app billing emulations. However, if you are not rooted, you need to rebuild the APK in which you want to use emulations and reinstall it.