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.
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.