Here's some things you might find helpful when navigating customs/duties for international shipments. Feel free to add information.

Remember: It's always recommended to check with your country's official policies on importing to understand applicable charges.

Canada Customs can sometimes be a little inconsistent when it comes to levying fees for duty, taxes, and processing/handling on inbound international package delivery. Couriers, such as UPS and FedEx, pretty much hit you every time for cross-border shipments; even on a $15 item, there are often fees involved. NAFTA helps with duties but does not cover all fees.[1]

Here's a quick breakdown on the various types of fees and how they are applied:
a) Duties - Proper NAFTA paperwork filled out by the shipper will result in no duties, but duties are not the only charge.
b) HST - NAFTA approved paperwork or not, HST is still applicable
c) Handing Fees - just to touch an item often results in the service charge being payable.

You should always expect (b)+(c), and sigh in relief if your item comes through charge-free. Generally, a mailed package of less than $100 value gets processed without charge .... generally. The current official exempt value is $20CDN (Jun.2015). Depending on the shipper and if they have filled out the proper NAFTA paperwork to exempt you from duties, (a) can range widely in the duty-% charged, based upon how the customs processor classifies the product.

1. ^Canada Border Services Agency website on importing by mail.

NAFTA : North American Free Trade Agreement
HST: Harmonized Sales Tax (a combination of federal and provincial sales tax)