I don’t know how much this will help, but may give you a start of where to look.
The error means it’s having trouble tracking down your shipping method to send the information to Google, have you made changes there recently? Did you define your shipping methods within the area in the Admin under System -> Configuration -> (Sales Section) Google API -> Google Checkout Shipping - Merchant Calculated…
Those directions could be a little confusing, so to get there you’d log into the Admin, click System and use the dropdown menu to go to Configuration. Within Configuration there’s a Sales section which includes the Google API option. Click that and then towards the bottom there’s three sections to set up your shipping for Google Checkout. Merchant Calculated is where the error is coming in, so I’d guess something isn’t set up right there. But basically use those three options to configure your shipping just as you have it in your store.
Google is a little different in that they have their own format for sending shipping information over so it’s tough to pull directly from what you already have set up in Magento, I would imagine that’s why they have you define it again here in the format that Google requires so there’s no confusion.
Here’s an article from the wiki that goes into the setup a bit, may help as well.
Hope that makes sense and good luck getting it set up.