How do I send them a receipt automatically with each payment?

You may create a recurring payment using our payment integration.

Here is a guide:https://www.jotform.com/help/166-How-to-Setup-a-Recurring-Payment-Option

You may then create an invoice to send to your respondents.

Guide:https://www.jotform.com/help/739-How-to-Generate-an-Invoice-for-Your-Payment-Form

Please check it out and let us know if you need further assistance.

This method does not work for recurring payments. There isn't an invoicing option.

My apologies, you are correct. The product list doesn't have an option to recurring payments.

May we know the payment processor that you would like to use for the subscription?

You may try to edit the autoresponder email and create an invoice.

https://www.jotform.com/help/430-How-to-Edit-the-Email-Template-for-Notifications-and-Autoresponders