$20 quick oscommerce modification

I want to limit registrations on an oscommerce site by only permitting certan postcodes - my idea is to change the Postcode registration field so that its in two parts - first part is a drop down list of available starts to the postcode and the second part completed by the customer. Post code must be collated in the database as a single field. Pretty straightforward? If so $20 is yours (I would offer more but I am very low on funds right now). Questions via PMB please.