Right after clicking ADD TO CART and go to the Shopping Cart screen, you will need to edit the Billing Information INSIDE THE CART. This is a one-time change that will not affect your User Profile on DreamHorse.
Look right below the EMPTY CART button within the Shopping Cart and in small print you will see:
Edit Your Billing Information Here
You can enter your Client's Billing Address inside the cart and this should resolve your issue.