I researched this idea a while back and found that Vistaprint has the automated sending service. I don't think it'll have the API approach of Sendgrid though.
There's also a whole industry of fulfillment houses for direct mail, but in my experience they're pricey and very traditional in their approach (sales-people, couriered proofs, no/printed analytics reports etc).
If you're going to do this regularly you can save a ton of postage by signing up for a business account with the PO and pre-printing the envelopes in a special format (although I always wondered if hand-writing would boost the conversion rate).
Also, try postcards rather than letters as USPS postcard rates + biz account will let you get your rates down to < $0.20/mailing - but sales letters may have a far higher conversion rate depending on your product, making them worth the extra postage.
There's also a whole industry of fulfillment houses for direct mail, but in my experience they're pricey and very traditional in their approach (sales-people, couriered proofs, no/printed analytics reports etc).
If you're going to do this regularly you can save a ton of postage by signing up for a business account with the PO and pre-printing the envelopes in a special format (although I always wondered if hand-writing would boost the conversion rate).
Also, try postcards rather than letters as USPS postcard rates + biz account will let you get your rates down to < $0.20/mailing - but sales letters may have a far higher conversion rate depending on your product, making them worth the extra postage.