Do you sell a subscription box on your Shopify store?
If so, Recurring Orders is the app for you!
Not only can you force customers to sign up for a subscription (instead of simply buying it one-time), you can also set a fixed shipping interval.
By default, customers will be charged for their first shipment right away, but you can specify the date of the month your customers get charged for the 2nd order, 3rd order, etc. If you want to charge all your subscribers on the first of every month, you can!