Hi Ram,
AdWords API tokens can be used in two ways:
1. You signup for a production token, go through the whole review process, and get your token approved. Once approved, you can use the developer token to make calls against production accounts.
2. Signup for a production token, but do not complete the review process. Your token will stay as "Pending review", but you can use that developer token to make calls to
test accounts.
In either case, you need to enter billing details and accept AdWords API T&C in your production account. You won't be charged for API usage, the part to enter the billing details is part of the UI flow as it exists today when accepting the T&Cs.
The form that you received is from Google requesting additional details to complete the review process. So that email should be interpreted as "If you want to complete the review for a production token, fill this form. If you simply want to use it for testing purposes, no further action is required".
Hope it makes sense?
Cheers,
Anash P. Oommen,
AdWords API Advisor.