post https://api.postgrid.com/print-mail/v1/self-mailers
Create a self-mailer. Note that you can supply one of the following:
- HTML content for the inside and outside of the self-mailer
- A template ID for the inside and outside of the self-mailer
- A URL or file for a 2 page PDF where the first page is the outside of the self-mailer and the second page is the inside
- Upload the aforementioned PDF file via a multipart form upload request