Payment Notes is no longer used by Google
This is where you can add information regarding payments and the ways you accept it. This can be very important, and can really affect the way your visitors view your products.

For example the Google checkout badge you see in the listing results, is identified this way. There is a whole section of Google's Help pages dedicated to this subject which you can see here.

The options added to the plug-in, allow you to specify which Payment Methods you accept, and also add in the Payment Notes as well.

I have found this useful thread in the Google Base forums that explains quite a lot about notes and the way it should be used. You can also add in the "PayPal" (excluding the "") into payment notes, to specify that you use PayPal, as you can see here on Google's Help Pages.

The Attributes look like this

<g:payment_notes>

Cash only for local orders.

</g:payment_notes>

Or

<g:payment_notes>

We Accept PayPal

</g:payment_notes>