1) The AMCAS letter service is wholly separate from the AMCAS primary application
2) You do not need letters for your AMCAS primary application to be submitted, verified or transmitted
3) Letters added to AMCAS after submission can be assigned to schools and will be transmitted in one business day
4) Letters are needed at the schools for your application to be marked complete and move on in processing
5) You can assign the letter to a school without the letter being submitted to AMCAS. That will likely stop processing on application until letter is received. You can assign the letter later but your application may be in processing and the letter may not be considered at that point