Domestic shipping only- by Categories/Products?

One of my product categories involves live plants, so they can't ship outside the US without lots of permits and other hassles.

I can't find a way to prevent customers from having those products shipped internationally while allowing them to buy from other product categories.

Any help?

What distinguishes these products? Are they always in a particular Category?

If so, does this mean that they always have the same master_categories_id in the products table?

If so, then you could write a function to see if any product in the shopping cart is from that master_categories_id and if there is one, either flag the Shopping Cart that the order cannot be sent internationally and prevent the order from getting past the checkout_shipping so that if I am in France I can still send the order to the US ...