They ship DHL, which is very thorough in finding everything they can charge you for. With my two purchases, they collected GST, PST and a brokerage fee, 6% duty on my Thai built ukulele, 11% duty on the case (China, maybe?). Depending where your ukulele/accessories are made (you said Opio, so same as me), will affect the percentages. Probably a US made ukulele will fall under the free trade deal, but I'm not sure.
Alternatively, I've got ukuleles (from Mim and others) through USPS/Canada Post which, at most, charged 5% GST and a small processing fee, occasionally, no fees at all. HMS is great, but you have to factor in these extra costs in your purchase.