A customer would sign up to a subscription which then credits their customer account with an amount to spend each month. The customer can then choose what they spend it on.
Note this is very different to a subscription to a specific product. We are aware there are lots of those extensions available. Also we don't simply want it to offer X% off all products for these customers. It's specifically the nespresso subscription model we're interested in - customers can spend their credits on any product on the site.