Convenience. Less hassle. No customs or duty/CRA paperwork. I don't know about cost (it's not cheap for me, but it's easy). I do what Toffle does, and I'm glad I heard his advice. In fact, I just dropped off 7 rolls of KR and KL this evening at my local drug store.
By hassle, sending a bunch of rolls over the border -- twice -- means it's an independent shipping effort which leaves me unaware of possible problems with handling along the way. By taking it to the drug store, I know that Fuji and Kodak institutionally handle the shipment, probably in tow with a manifest of other customer orders. They have a system, it works, and I know that if there are any problems with processing, then it's limited by Dwayne's performance; the way I handled the film personally; and, if relevant, whether I bought expired stock from someone else (as a uni student, I'll happily use what I can afford).
Having the assurance of transit handled professionally is worth it.