Yes! You may send a letter as an attachment with your check. When creating a check, you would need to pass your letter information in the attachment parameter. The attachment can be: an HTML string, the template_id of a saved HTML template, a remote URL, or a local upload of an HTML, PDF, PNG, or JPG file. The attachment pages will be printed double-sided in black & white and will be included in the envelope after the check page.
Please see find additional information on the attachment parameter here.