Stores is free for one person on one list. That covers adding, checking off, repeats, store tags, due dates, task lists, colors, dark mode, and a home-screen widget.
One upgrade unlocks the rest: share a list with anyone, keep more than one list, add by camera, and pick a premium app icon. Comments are free on any shared list, or come with the upgrade on a list that's just yours.
It's a one-time $4.99, with no subscription. Your partner joins for free, Family Sharing covers it, and you can restore it on a new phone any time.