Olaf (a member of this community) has been kind enough to share his code for how to implement in-app purchases with CoronaCards for WP8.
Here is the link to the github repo that has his code (and that has been commented):
https://github.com/olomorolo/CoronaCardsWP8_IAPguide
This uses the Lua/Native bridge that is now enabled and allows you to implement any feature natively and hook it into the Corona code.
Thank you Olaf!