Groceries on Amazon?

I have Orville Movie Theater Butter Popcorn on my grocery list- the only brand we really like, and we it on a regular basis. I noticed that on Amazon, you can "subscribe" to a grocery product and have it delivered on a regular basis- every 1,2,3, or 6 months, I think. Anyway, the popcorn we like is $18.42 for 12-3pks. That comes to $1.53 and a half cents per 3pk. (No shipping when you "subscribe"). I can OCCASIONALLY get it cheaper when I have coupons, but often pay higher prices- 2-$2.50/3pk since we eat it on a regular basis. I am really considering doing this, but I'd like to hear form others and see if anyone else does this- if not with popcorn, then with other products you consume regularly. Is it really a good deal, or will the prices jump with no fair warning?