While VisitorSecure does have state restrictions, you can still purchase the plan with your parent’s home country address at any time. It is also worth noting that if you are purchasing the plans before they arrive in the U.S., you can still use the New York address accordingly. When you select New York as the state, the application will ask if the insured has already arrived in the U.S., If the answer is yes, then you can use their home country address. In either situation, you can use any credit card with any billing without issue.
That being said, you may also consider a very similar plan- Safe Travels for Visitors to the USA, which does not have any state restrictions at all.
Insurance rates are regulated by law. You can't find the same insurance plan for a lower price anywhere else