I infer you're referring to our checkout page for card-based direct payments using PayPal Payments Pro. Bear in mind that the Shipping address there locks-in the Country, State/Province, and Postal Code the buyer entered in their shopping cart before proceeding to checkout, so if they enter a Billing address where any of those differ and try to tick the box for "Shipping same as Billing", that would produce an error. If they wish to change any of those Shipping address details, they'd need to go back to their cart and change it there, as that may affect their shipping charges calculated in the cart.
If that doesn't seem relevant to the affected buyer's situation, please email us with a link to your sales page and the complete address the buyer had trouble entering, along with any other relevant details they may have provided, such as the exact error message they received:
https://www.e-junkie.com/ej/contact.htm
