Per the preorder confirmation email;
"You will not be charged until your order ships"
Also the play FAQ's say;
"Once you place an order, charges generally post to your credit/debit card after your order has shipped"
Regarding the credit, you won't see that until you access the play store from a device identified in the build properties as the nexus 7 tab. Thats speculation but per the
FAQ;
"just open the Google Play Store app on your Nexus 7 or visit play.google.com, and start shopping"
When you log into the play store from a device it logs the device maker and model (among other info) which is used to determine application compatibility. The play store keeps a record of any device your google name uses to access the store. You can see them if you use a web browser to go to the play store, my account, settings. My guess is once you hit the play store with a device that identifies it's self as the N7 the credit applies.