Can one-time payments be added to future subscription billings?

Yes, we understand that some merchants would like to bill additional single purchases alongside a customer's subscription renewal. The best to do this within the UI would be to visit the customer's account details page within the application. From there, you'll want to click on Add Charge near the Charges and Credits section of the account page. Once that's been done, you can fill out the charge details which will impact how the charge is shown as a line item on the created invoice.

Depending on if you have credit invoices enabled or not, you'll either be able to invoice the charge immediately or select to bill the charge adjustment alongside the next subscription renewal. Once the charge adjustment is created, it will automatically be pulled into the next subscription creation or renewal that occurs on the account and bill as a new charge line item.

Similar behavior can be handled through the API as well by utilizing the Create Charge call:

https://dev.recurly.com/docs/create-a-charge

Once that charge adjustment is created, it will function the same as if it was created on the account. The adjustment will sit in a pending state until either a new subscription purchase or renewal occurs and then the adjustment will automatically bill alongside the new subscription invoice.