There was a PDF attached to my secondary email from MU-COM with the directions for the LOR's. Maybe you have one too?
I can't copy/paste because it is PDF, but it says they prefer electronic delivery (ex: Interfolio) and your AACOMAS ID MUST be included. It also says specifically do not send via USPS.
You need 3 letters: physician, science professor, pre-med advisor (or an additional science professor if you don't have an advisor).
The website explains: In the absence of a letter from a pre-med advisor or committee, we will consider the faculty letters. However, it is strongly recommended that you consider offering a biology professors evaluation as part of the packet. Since biology is the most relevant of the sciences to medicine, assessment from a biology professor regarding your ability to handle medical school level work would be of great value.
Additionally, make sure that your application explains the absence of a pre-med advisor recommendation.