Google Checkout cannot be used for items with prices less than $1.00.

If you have items in your feed at that price, make sure to strip out Google Checkout from the payment_type field or else those lines will be rejected by Google Base.

Obviously this is a small issue, but knowing this can save you the hassle of waiting for Google Base to return the error and re-submitting. Hopefully in the future Google Base will be smart enough to automatically rip out Google Checkout from the payment_type field if there are other payment types listed.