When I ship by priority mail via USPS, I see it typically take one day to the Border, but tracking then stops and no one knows when it will be delivered, could be in a day or in 6 weeks.
B&H and Adorama have contracts with the shippers that typically cover the cost of customs handling. Its lucretive for a shipper to ship several truckloads a day from a company like B&H, so the discount is very hefty. amazon has warehouses at key locations around the US so they are never more than two-three days from the customer, but if it comes from a distant warehouse, they eat any extra cost to give 2-3 day shipping..