Charging Different Base Shipping in eCart5 (i.e., one for U.S., one for Canada, one for Rest of World)

I'm a law professor, not a programmer, so I see references that glance at this topic but none is as explicit as I need it to be.

In what file and at what line number should I insert what .php language that would enable me to charge one base shipping rate for U.S. customers, a second base shipping rate for Canadian customers, and a third base shipping rate for everyone else? I suspect that it should go in eCart1_PHP.php but really don't know, and if I start adding some "IFs" on my own I'll tank my whole site, which is otherwise completed.

If it's easier, I can make the change using the dialog box in the eCart Object that pertains to shipping, but none of the suggested rules in the drop-down menu has anything to do with the country of the customer.

Please help! Thanks so much.