Interfolio is one of the letter delivery service you can use to transmit your letters of recommendation to AACOMAS and/or AMCAS. There are other letter delivery service such as VirtualEvals but from my experience and consensus on SDN, Interfolio seems to be more user-friendly. I also like Interfolio because those letters are stored in the event you need to reapply to medical schools. And how it works is, basically you send your writer a request via Interfolio and your writer can either upload their letter directly to the site or alternatively, you can print out a cover page that gives them instructions exactly how to mail their letters to AACOMAS.
People needing to use the service, including myself, have been worried over the past week because Interfolio was unable to accept letters from applicants for AACOMAS applications. And yes, I think you can have your writers directly upload their letters to AACOMAS.
Hopefully this helps!