My spouse sells gift certificates for her business.

It's a small volume.

Whenever someone pays for a certificate on stripe, she receives the mail and she sends the certificate to the customer's email that initiated the transaction on Stripe.

The certificate is a simple word file with a certificate number she increases manually before printing it to pdf.

You could write an api that's called by stripe whenever a transaction occurs, validates the transaction with stripe, prepares the pdf and sends it to the customer.