Project for PHP experts

Simple job for an expert Requirements: We currently utilize ActiveCollab as the core of our Project Management System. We've completed updates, one specifically allows clients to click a "Make Payment" link on an invoice page which directs them to [login to view URL] to pay for an invoice. We'd like to take the google integration one step further buy allowing the clients to make their credit card payment from within the ActiveCollab platform using Google Checkout as the payment gateway (Google Checkout API). The user would click the make payment link and the web form for their credit card information would be shown and then processed through Google. No credit card information would be saved to the system. The necessary web services would also be required to accept notifications back from google with regards to successful transactions and failed/canceled transactions. The web services would update the invoice status as necessary (e.g. successful transaction would update the invoice as paid within the activecollab system). The experience needs to be tightly integrated with the ActiveCollab platform so an expert understanding of ActiveCollab is required. Proper user flow must be maintained at all times.