These two free addons for MP shipping plugins allow more options for flat rate shipping. One is for flat shipping to the individual states in the US. The other is for flat rate shipping to the US split up into 6 zones.
Here’s what you do.
(1) Drop the PHP file into the /marketpress/marketpress-includes/plugins-shipping/ folder
(2) Visit Store Settings >> Shipping inside your WP dashboard. The “Flat Rate Intl” theme should show up in the dropdown box.