Amazon. Is. Amazing.
We worked with Shipwire for over a year and it was a nightmare.
Customers expect things to leave the warehouse quickly... and then get to them fast. Other fulfillment houses have weird "cutoff times," a limited number of warehouses, and less amazing arrangements with shipping companies Nobody can come close to doing those things as well as Fulfillment by Amazon.
Often our customers in large cities get next-day delivery even if they don't pay for it. They get things on Saturdays *and* Sundays.
With Shipwire, an order placed at 9 a.m. on Friday wouldn't leave the warehouse until 9 pm Monday. Then they'd have to wait many days for the shipper to get things to them.