Buy Me A Coffee has a Monthly Subscriptions feature which allows creators to enable 'monthly' option on their Buy Me A Coffee page.
Click on the Enable Monthly Support checkbox on the Coffee Options page in your dashboard.
Your page will now display the Monthly tab, following which you can specify the number of coffees to subscribe for. You can pay using all major credit/debit cards, or you can subscribe using PayPal.
Here is a screencast of this feature.
This feature is intentionally simple, but we plan to add more features in the upcoming months.