Consider if capture should utilize the `_links["captures"]` part of the authorization response; then the client doesn't need be updated if endpoints move around. (Pointy finger to http://en.wikipedia.org/wiki/HATEOAS :-))

It would work well with `#charge` but could be inconvenient if you capture separately (since than you'd need to store the entire endpoint string rather than just the `transaction_id`), however ... well ... yeah, depends on what you prefer.