Hi @Samir_Hembrom,
You should be able to test the Billing API with a non-client transfer development store created from the Dev Dashboard.
There is a known issue specifically with Plus Development Stores, where you are unable to approve test billing charges.
Our developers are aware of this issue and are working on a fix for this at this time, and you can follow this thread for more updates on when it is fixed.
In the meantime, as the issue is only happening with Plus Development Stores, you can work around this by creating a new Development Store for testing, and select any other plan for testing, other than Plus.
I’ll mark this thread as closed, and if you have any follow up questions you can reach out in the existing thread I linked above.