Subscription electronic toothbrush / replaceable heads. I used to use a site called something like One Dollar Toothbrush that sent a monthly replacement, they were eventually acquired and prices went up, so I dropped the service – but I still use DollarShaveClub and would be interested in such a subscription service. The startup costs are a bit high for my tastes ($25-$40 to get the electric toothbrush) but $5 every three months doesn’t seem outrageous for refills.