It was confusing to me at first too, so here is a super simplified diagram for you:
Option 1:
Letter of Rec Writer (Prof/PI/etc) --1a---> Interfolio/Private Letter Service/School Letter Service --2---> AMCAS
Option 2:
Letter of Rec Writer (Prof/PI/etc) --1b---> AMCAS
Key
1a: Forms from interfolio/school service/whatever letter of rec collecting service is given to the letter writer, and they submit it to that service for you
1b: Forms from AMCAS are printed out and given to each individual letter writer, and they submit it in to AMCAS for you
2: Forms from AMCAS are printed out and given to the private service/school service/interfolio, and they will submit all your letters to AMCAS for you
Pros/Cons
The purpose of a letter service is to organize your letters on your behalf, and to store them for you should you require them again. Some services will give you a couple years so you can reuse those letters if you need to apply again. Many will also double check that everything the letter of rec writer submits is correct for your application (right name, program to which you are applying is referred to correctly)
Option 1
Pros
- Stores letters if need to repeat application cycle
- Some make sure everything is signed, your name is correctly written on the actual letter, and small things like that
Cons
- Possibly will force all letters to be sent as one packet (can't pick and choose which letters for which schools if need be)
- $
Option 2
Pros
- Can choose which letters to send to which schools with AMCAS directly
- Cheaper
Cons
- Loss of benefits from having letter service (see above)
Hope this helped, and good luck