Passing billingDetail couples the hosted cashier to the order's billing country: it then offers only that country's payment market and the customer cannot switch. The country that applies is the one on the finished order, not the one you sent; a country outside the payment markets we cover applies no restriction. If the coupled market offers none of your enabled payment methods the order cannot be paid. Omit billingDetail to leave the cashier unrestricted.
Types unchanged — no code migration.