I used Postful for years, and moved to PostalMethods when Postful was sold a couple of years ago.
I quite like PostalMethods -- it's even simpler than Postful was (Postful required the to address in the subject line of your email). They help you format your letters so the addressee will show up in the right location for a window envelope... then once you're all set up, you just send PDFs to them via email, and each email is another letter printed and posted.
They have an API as well (I think that's actually their focus), but I've never used it -- I only need snail mail at low volume and manually-created, so email is perfect.
I don't have any need for EasyPost, personally -- because I'm doing principally US mail while I'm living elsewhere in the world.
We've used PostalMethods with both Quickbooks (invoicing clients who don't take email for invoices; Quickbooks emails the invoice to PostalMethods, who prints and mails it for us) and for our clients (using the API to send letters and postcards).
