PostalMethods Web-to-Postal is what you are looking for.
Check out our Postal API section containing all the data you need to quickly send letters and postcards directly from any application. The Reference Manual of our SOAP Web Service API and many Code Samples will help send your snail mail quickly.
We also offer a free development account and free, quick and unlimited support. Pricing model is pay-as-you-go. Purchase a prepaid card from $10 to $10,000 and enjoy volume discounts.
The letter recipient would never know you didn't mail the letter yourself because there is no indication of PostalMethods anywhere. We use a #10 double window envelope and the postage is a business indicia.