That is because Zulily is a flash sale shopping site. It works like this:
Supplier and Zulily have a chat about available inventory, and agree on a discounted price. Zulily pay the supplier less than the usual trade price, but offer a good sized order in return, so everybody is happy.
Zulily runs the sale.
Three days later the sale ends.
Zulily puts all the products that have been ordered into a spreadsheet & sends it to the supplier as a Purchase Order.
The supplier checks it, and issues an invoice.
Zulily pays the invoice.
The supplier packs everything up and ships it in bulk to Zulily's warehouse.
Zulily checks it.
Zulily gets out all the customer orders, and picks & packs everything, and ships it.
The supplier is happy, because they sold a whole bunch of last year's stock AND got paid up front. Zulily is happy because they made a nice cut of the sale with no risk. The customer is happy because they got 20% - 60% off retail, but the DOWN side is that the customer has to wait about 2 weeks longer than usual because of all the shenanigans.