Jedenfalls rät google uns auch in der Google Checkout Buyer Help bei Änderungen den seller zu contacten.
Shipping

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Add / Revise Shipping Addresses

You may add shipping addresses to your account or revise them at any time. Changes cannot be applied to existing orders; you will need to contact the seller to request shipping address updates. If you prefer to cancel and reorder, you can check your order receipt for the option to cancel. If the option is no longer available, contact the seller with your cancellation request.

Wie gesagt...vielleicht klappts ja noch.