You'll _really_ want to avoid USPS for US -> Canada shipping, it takes at best 2 weeks, at worst 3-4 weeks. DHL is the best option, they will ship US to Canada in a day via their standard services, but they can be expensive. UPS typically averages 3 or 4 days, I have yet to extensively try FedEX for US to Canada shipping.
Canada to US shipping via USPS is typically faster, averaging 2 weeks. So you'll probably want to completely avoid USPS/Canada Post, since that would eat into your profits. For the extra cost in non-USPS shipping methods, you'll make up for it by the additional time you can rent out the lens to someone else.
Customs wise, yea, who knows what will happen, I'd be interested in knowing what happens too.
Regards.
1DXII, 3x 1DX, 5DSR, 1DIII, 1DsII, 6D, 7D, 50D, 30D, Rebel XT; 12-24, 14/2.8 fisheye, 20/1.4, 17-40/4L, 24-70/2.8L, 18-50/2.8, 3x 70-200/2.8 IS, 24-105L, 24-70/2.8L, 50/1.8II, 28-135IS, 85/1.2L, 135/1.8, 200/1.8L, 120-300/2.8 OS, 300/2.8L
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