AMCAS has it's own letters service that AACOMAS does not have. Interfolio is used by all osteopathic schools though and is preferred by AACOMAS for applicants to use.
You only send letters to the schools that request them when they give you their secondary to complete. The letter requirement will vary from school to school.
I did my UG at UCD so corresponding with the letters services that was offered to us by the advising would have been difficult since I returned to Portland after graduation. But interfolio has been great to me so far and I am at least comforted to be notified immediately when letters have been updated and I can personally see/send what letters I have to wherever I need them to go.