I checked my old notes. I thought the store owner would set shipping an tax settings in his Google Merchant Center settings.
But you want to override them.
I'll add this feature on next Wednesday.
You will have 3 new parameters
shipping - country
shipping - service
shipping - price
If you set them your feed will look like
<g:shipping>
<g:country>DK</g:country>
<g:service>Standard</g:service>
<g:price>10.00 DKK</g:price>
</g:shipping>
Do I understand you correctly?