They really are a good idea IMO. The idea is you put the button next to the garbage bags, detergent, etc. Hey, the detergent just ran out, click, ordered and done. Very convenient and eliminates the problem of forgetting to add <whatever> to the shopping list.
One might object that forgetting to add <whatever> to the list is really not a terribly large problem, and you would be right- but $5 is also not a terribly large sum.
For me personally, there's usually a gap in time between when I notice <whatever> needs to be replenished and when I actually do it. Hence, the possibility to forget. Having the button co-located with the item in question essentially eliminates that problem. And $5 is worth it for me not to have to make a special trip to the store when I really DO need <whatever>.
