If you are accepting payments through the Stripe payment gateway, you can toggle on the option "Include Stripe receipt email in the checkout" under Configure → Payments to have Stripe automatically send a receipt for the purchase.
Sending Receipts from Paperform
Paperform supports sending a standard payment and submission summary as text in the body of an Email Response, as a PDF submission summary attached to an Email Response, or as a Custom PDF. Here's more information on these:
1. Choose "Receipt" as the content of the email
2. Choose the "Custom Message" option, you can optionally append the receipt to the end of your custom email.
3. Attach a Custom PDF, that you'll need to have already created via After Submission → Custom PDFs.
Gotchas
Passing information from your form to the Stripe receipt is currently not supported.
Sending Stripe receipts is only available for the Stripe Credit Card checkout, if you are using a different payment source (e.g. iDEAL pay), then the email address may not be captured.