Massdrop made items that are in-stock, they are on a cyclic drop period. It refreshes after 24 hours and they try to get the order shipped to you as soon as possible. The button will say "buy" in this mode. You just have to wait for it refresh
If they are out of stock, the manufacturer needs to make more and ship them to the Massdrop warehouse in NJ before they are shipped to the customer. The button will say "pre-order" in this mode.