oscommerce customisation

I have a client who sells fuel oil to private and commercial customers. He needs to have customers register as either Commercial or Private and, more importantly, if they are Private customers they pay 5% VAT and if they are Commercial they pay 17% VAT. We are using a customised installation of osCommerce 2.2 Only bid if you are confident that this can be achieved QUICKLY and EFFICENTLY. Questions via PMB only please.