Ask HN: Recommendations for an online service that prints and mails postcards?

I'm trying to find an online service that can print and mail semi-customized postcards as part of a direct-mail marketing campaign.

This is all within the United States.

I have a database of several thousand recipients. I also have a program to automatically generate globally unique coupon codes.

Can anyone recommend a webservice, preferably with an API, that I could use to automate the printing and mailing of these postcards?

Thank you!