You can use the Zone Rate zones shipping module and set it up for:
US
99.99:3.50;10000.00:0.00

CA
10000.00:25.00

and later for the Rest of the World use:
00
(rates as needed)

The 00 means anyone not otherwise defined ...

If you need more than 3 Zones, then you would edit the file:
/includes/modules/shipping/zones.php

and set the number of Zones to how many you need ...