I tweaked a couple of things on the cart, and added First Class International shipping as an option, so if you're interested in a book or two or a shirt or three, now's the time.
It also turns out that a few of the people who had gotten an absurd number for shipping, had left it on the default "Priority Mail", rather than selecting "Priority Mail International" from the menu.
That wasn't the whole problem, but it did still jack up the shipping cost unnecessarily.
That said, even 'International' is still steep- not my cart's price, but the actual straight-from-the-Post-Office price. Most of my overseas and Canadian stuff gets shipped either Priority Mail International Flat Rate (for $16.95) or First Class international (which starts at $10.95) so even the "corrected" prices are occasionally off.
But as I said below, and in the quick blurb on the checkout page, for the time being, if you place an order and the shipping cost comes up wrong, I've been refunding the difference.
Certainly not the optimal situation, but it'll get us through this run, and I'll do more tweaking to the cart software before I launch the next.
Thanks for bearing with me, and thanks to those that have given me some useful feedback.