That's the best advice I've picked up from this website, actually. And in this case, I did prepackage, measure and weigh the parcel. I'm just not sure why there was a discrepancy between what I calculated using Canada Post's website versus what I ended up paying using PayPal's integrated Canada Post service. Although I can't envision myself as a big time re-seller, it was a neat experience selling something and turning a profit, even if it's only a paltry $5 sum!
Thanks for the advice, @Zelgazra! I'll definitely use this experience to do a better job of calculating my shipping fees to maximize my profit.