http://5rzkisd745el4yr7krh5qc4f5sgblvg4i5qtuxivfgxcxtr65ejd7gyd.onion/help
It runs on a fixed timer unrelated to visitor activity, and browsing the catalog or a card page never causes one: those read the last cached rate, or simply omit the estimate. The one exception is that a checkout will refresh a stale rate, because a real order needs a real rate to lock — so under a narrow condition a checkout can coincide with that request.