osCommerce: UK Shipping/Tax calculation for Google Checkout

We are in UK and we have a heavily modulated oscommerce site. The official Google Checkout seems to be installed and running OK till the stage where it call back for Shipping/Tax info. Then a message will be displayed to the shopper that we don't ship to his/her region. We are using a table-rate shipping methods based on Tax Zones. The countries/Region where we can ship our goods to are classified into 5 Tax Zones, and the countries/regions where we will not ship our goods are not listed in any of the Tax Zones. We have three different shipping methods (Express Courier Shipping, Royalmail Special Delivery, and RM 1st/Standard Parcel). Each shipping methods have a set of table-rates for each Tax Zone, for example - the Express Courier Shipping and RM 1st/Stand Parcel have the following shipping table-rate: Express Courier Shipping: Zone 1 Countries UK Mainland Zone 1 Shipping Table 1:4.24,500:5.00,1000:5.94,6000:9.98,10000:8.49 Zone 1 Handling Fee 0 Zone 2 Countries Norther Ireland Zone 2 Shipping Table 1:5.94,500:6.79,1000:7.64,6000:8.49,10000:8.49 Zone 2 Handling Fee 0 Zone 3 Countries European Countries Zone 3 Shipping Table 1:8.49,2000:17.00,4000:25.51 RM 1st/Standard Parcel: Zone 1 Countries UK Mainland Zone 1 Shipping Table 1:2.54,500:6.79,1000:8.49,2000:17 Zone 1 Handling Fee 0 Zone 2 Countries Norther Ireland Zone 2 Shipping Table 1:2.54,500:6.79,1000:8.49,2000:17 Zone 2 Handling Fee 0 Zone 3 Countries Channel Islands Zone 3 Shipping Table 1:2.54,500:6.79,1000:8.49,2000:17 Zone 3 Handling Fee 0 Zone 4 Countries Friend Countries Zone 4 Shipping Table 1:6.98,500:9.98,1000:12.98,2000:39.98,3000:49.98 Zone 4 Handling Fee 0 I think it might be necessary to change some of the official Google checkout codes to make it working properly with our shipping methods. We are looking for somebody with experiences in osCommerce and Google Checkout to do a quick fix so the tax and shipping can be calcuated properly for those customers using Google Checkout.