If somebody is a Seller, not a Seller+ & uses google as a payment option, as far as I know it shows on their listing, but as integration isn't available, they invoice the way the OP wants to.

On the My personal details section it says
Google Checkout Merchant ID
Google Checkout Merchant Key
Let your buyers checkout with Google Checkout Seller+ only.
Both elements required.



So I'd imagine, ticking google checkout as a payment option for your listings, but NOT filling in the google merchant information in My Personal details would do the trick.