I think there are ways you can get repeat purchases without the subscription model. Yes, having the CC details stored (actually, having some kind of token stored, ala Stipe) would make repeat purchases easier.

There are probably other things you could do to speed up the process. Maybe a sticker (or mug) with a QR code that leads to the purchase screen. Something that's always in front of my face that I can scan when I realise I'm down to my last cup.

Subscriptions are fine, and there's no way you should drop that model, but having both options would be way better.