I need to dynamicaly remove tax during checkout for business to business customers in Magento. I have a module which asks for a VAT ID during checkout and checks it against the European Online VAT-ID Validation Service. If the VAT ID is valid, the tax should be removed during checkout.
The module already assigns new customers with valid VAT ID to the according business to business group in Magento. Customers who order for the first time and create their account during that first checkout, get charged VAT (even with valid VAT-ID).
I basically need to know, how this can be done, which methods involved, etc.