If they only have one shipment address make this the address on the card - then set the appropriate invoice relationship up if they go to another address - no code is required.
On the invoicing tab of the customer the first field is the bill-to customer, here you specify the customer the bill goes to and all the invoice address information is taken from the address on this customer card. You shoul dbe aware though that the “sales” are as standard pushed here as this is where the financial transaction takes place.
You put the ship to address in the address fields on the general tab of the customer card.
I have assumed from your request that you have one fixed shipment address that is different from where the invoice is sent, and hence my suggestion, otherwise you would just use Navision as you always would with one address.