Switching Google Account for payment on Supporting the game?

I'm trying to buy a support tier on the Google Play version of Shattered PD. I have two Google Accounts linked to my phone. Account A, which I used to install the game and is linked as a Google Play Games Account to the game (and that has Google Play credits available) and a second one - Account B - with no Play store credits, which shouldn't be used with or be connected to the game in any way.

When I hit the IAP of one of the support tiers, it always opens up the payment method selection of Account B. I already tried to uninstall the game and install it with the correct account (which helps for other apps in this case) and also reconnected Google Play Games to make sure it's also tied to account A. But the only way to pay is account B. there's no Account selection in the payment dialog.

Has anybody an idea, how I can get the game to use Account A for IAP?!