How do I use Google Checkout with Google Product Item Feed

Question

I have connected my Google Merchant Center account to my Google Checkout account, but how do I set Google Checkout as the primary method of payment for a Google Product Item Feed?

At the moment I have

<g:payment_accepted>Google Checkout</g:payment_accepted>
<g:payment_notes>Google Checkout Accepted</g:payment_notes>

But that doesn't seem to work.