Why is Guam and Virgin Islands shipping the same as next door?

[musicsubmarine]

 I sell some reasonably heavy items 20 to 40 pounds, and I usually do flat rate shipping to the US with a surcharge for Hawaii Alaska in Puerto Rico. If somebody from Guam or the Virgin Islands buys it, they just pay the regular flat rate without being charged the surcharge, even thought the cost is much higher than alaska, PR and HI. There's no way I can find to block them either, does anybody know where around this or what's the solution for this? I don't want to use calculated shipping. Any advice would be greatly appreciated.

Thanks, Paul