Ask HN: Easiest way to add membership to a site?

I'm looking to add membership functionality to a project and am curious what people out there use. In particular, I'm looking for a simple way to allow members to choose from a selection of plans that I define. In addition, it would provide membership management tools, user flows for upgrading/downgrading/cancellation, dealing with expired cards and other common functions. Doesn't need to do it all but if prefer not to have to write everything myself if possible. Thanks for any suggestions or ideas.