What is the Cheapest/Best Shipping Service?

[Freelancer-Mountaineer]

I need to ship a pair of sunglasses back to an eBay seller because they were defective and need to get repaired. I believe the seller will reimburse me for the cost of shipping but I can't be sure. What's the best and cheapest shipping service? UPS? Fedex? Canadapost? I want it to get there immediately so it can be sent back quickly and also spend less time in transit. Thoughts?

[parry]

cheapest is canadapost