I second this. Just do it yourself with a cron job. It can be really simple in the beginning, and very flexible later when you need it. As your business grows, your charging can quickly get complicated. Think custom subscriptions with conditional coupons, multi-currency, custom pro-rating, special discounts with specific conditions. With your own charging solution, you can do all these with ease. But if you off-load billing and charging schedule to an external service (and use webhooks or reverse APIs), you will quickly discover you're pretty limited.