Ya buying stuff online is hard for me too since my mom won't let me use her credit card but you could try buying a giftcard from amazon and using that.
Maybe those websites like swagbucks,zoombucks,or prizerebel will help I tried but it takes too long.
Or you can see if your phone has an app for amazon coupons.I use one that gives $3 worth of an amazon giftcard. It's called 'smart phone mate' for android phones but iphones have one too I heard.
Until I'm convinced that black castor oil is better I'll stick with the plain laxitative one in stores.