> I know stores like Book Depository offer free shipping internationally on books as cheap as $10…

Businesses can get better rates for shipping than individuals.

Also, from the USPS web site:

> First-Class Package International Service

>Affordable Lightweight Shipping

>First-Class Package International Service (FCPIS) is an economical way to send lightweight packages to about 180 countries. Packages can weigh up to 4 lbs (64 oz) and contain goods up to $400 in value. Other restrictions may apply based on the destination country. First-Class Package International Service includes Electronic USPS Delivery Confirmation® International (E-USPS DELCON INTL®) when you ship online to select countries, although certain limitations may apply. Prices start at $14.85.