Disabling auto-renewing charges on your Prime subscription is needlessly clunky and misleading. You can safely "End membership" and "Cancel My Benefits" without losing any remaining paid time on your subscription.
*INSTRUCTIONS*
1.) Navigate to your Account home: www.amazon.com/gp/css/homepag...
2.) Click the Prime button.
3.) Click "Update, cancel and more"
4.) Click the "End membership" button.
5.) Click the "Cancel My Benefits" button.
6.) Click the "Continue to Cancel" button.
7.) Click the "End on ..." button.
Go ahead and leave it off for a few months. You can always re-sub at any time.
But you might be surprised at how little you miss it.