Squarespace Email Campaigns - Email, but pretty.
Lob - A simple API to integrate print & mail solutions into your applications.
PostGrid - Transform your Offline Communications. Use our fully-documented REST API to send personalized letters, checks, postcards and improve address accuracy.
Growmail Automated Direct Mail - Growmail enables businesses to send letters and postcards instantly through Automated Direct Mail.
PebblePost - PebblePost invented Programmatic Direct Mail® to transform real-time online activity into personalized, dynamically rendered direct mail.
AmazingMail - Business print-mail solution