Party planning can be fun if you have the time for it and happen to know what you’re doing. For the rest of us, it can be a daunting, time-consuming endeavor, one that requires visits to numerous websites, in-store visits when those products invariably don’t arrive in time, then return visits to pick up those last items that you could have sworn you’d thrown in your shopping cart but did not. Enter Coterie , a nine-month-old, New York-based startup that was incubated with the help of the investment firm Female Founders Fund and that is assembling party kits that it’s delivering to customers’ doorsteps, for everything from birthday parties to baby showers to friendversary get-togethers. Just tell the site how many people you expect, whether it’s 10 or 50, then pick a kit. For example, the “lux” version of its “shine on” package — which could pretty much suit any occasion — comes with glittery plates, metallic flatware, votives, string lights, gold paper straws, dressed-up paper cups a...