Given how (sometimes poorly) automated eBay is, I'm not sure how you'd expect the software used by the Global Shipping Program to recognize that your address is a post office box.
In addition, the terms don't say that the charges "don't apply" if the item is sent to a P.O. box but, rather, that you're not supposed to use a P.O. box as your shipping address. Quite a different animal.
The reason why the Global Shipping Program terms don't allow buyers to use P.O. boxes is because the item isn't always delivered by the post office. Having said that, in Canada, the post office is often contracted to deliver items from the Canadian receiving hub and the buyer and there's a chance your items may get through and reach you anyway. I'd give the items about three weeks to reach you and if they haven't, file claims of non-delivery to recover your funds.
As for your concerns that the Global Shipping Program charges are expensive, that's true, given that you're paying for two sets of shipping charges. The first charge is what the seller charges to ship the item to the Global Shipping Center in Kentucky, and the second charge is what PitneyBowes (administrators of the GSP) charges to have the item shipped from Kentucky to you. Having said that, items that are shipped directly through the postal system can be more expensive than the item price, too. It's the nature of purchasing from outside of the country. It's just that sellers have more control over how much they charge for shipping directly to Canada and tweak with their listings accordingly.
