Hey guys so I'm going to ask my teachers for letters when the application opens up (May).
We're a small school so they won't have a hard time writing a letter for me.
With that being said, if I am going to ask them in May, will it be useless to use interfolio?
Also if I don't use interfolio, that means they have to send their letter twice right? one to AACOMAS and one to AAMC?
I was in a similar situation as you and I had my letter writers submit letters to Interfolio in early June, and I submitted my app later in June. I would recommend contacting your letter writers ASAP as opposed to waiting until May, if possible. It's generally recommended to give your writers 2-3 months of notice prior to when you need the letter.
Interfolio made the process super easy for my letter writers because Interfolio sent a link via email, and all they had to do was click and upload their letter directly to Interfolio, and then I could determine which app system I wanted to send the letters to (since I sent all of my letters to AMCAS and only sent 3 to TMDSAS). To answer your question, if you decide not to use Interfolio, your letter writers will go through the process of submitting their letters to both application services, and the submission process is different for both.
Also, Interfolio has this great service where they check the letter to make sure it's written about the correct person, is on letterhead, and is signed. Two of my letter writers didn't sign their letters, and one didn't write it on formal letterhead, so Interfolio contacted me about it and I was able to get my letter writers to upload modified letters. I can't speak highly enough about Interfolio! It costs like $40-50 for a yearlong prescription, but it's worth every penny!