Well, not as bad as I thought. This is what the USPS says about the rate of the SMALL box:
The Priority Mail Small Flat Rate Box has a retail shipping price of $4.95 — one price, regardless of weight, up to 70 lbs. for U.S. addresses. The international price, for up to 4 lbs., is just $10.95 to Canada or Mexico and $12.95 to all other countries, with savings of 5 percent for customers who ship online.