The University of Iowa does not offer Committee Letters or Letter Packets currently, so Individual Letters is the way to go!
Things to Consider When Choosing Letter Writers:
requirements of each school you're applying to (see below table)
how well they know you, your abilities and qualifications
This table is not exhaustive, and if you have questions, it is best to reach out to the medical school directly for requirements (see contact info column)
How to Ask for Letters of Evaluation
Have an unofficial transcript, draft of your personal statement and resume prepared to offer your letter writers for further context of your application.
Tell them WHY you want a letter from them (i.e. did you feel you made great strides in their class, are you trying to show the empathy you exhibited in your volunteer work, etc). This will help them know how to add new stories and insight to your application.
Give them as much notice as possible! If you want to have their letters submitted when the application opens for submissions in May/June, ask for your letters in the spring (around spring break is a good time to think about it)!
Let them know the timeline you want them to submit by and REMIND THEM of that timeline as it gets closer (1 week before, etc).
You will want to ask for your letters before AMCAS is actually open for submissions if you are planning to submit your application as soon as possible. This means you won't have the AMCAS-generated "Letter Request" paper to give to your letter writers. Make sure you get this to them as soon as possible, though, because it includes numbers AMCAS uses to identify your letters and pair them to your application.
It can be scary/strange to ask for a letter of evaluation the first time, just remember all the faculty at the University of Iowa are there to support you and those who know you well want to see you succeed!
In my experience, I found that it was best to ask at least a month before you plan to submit your application. It is also important to have built up some sort of relationship with the person you chose as a letter writer, so that they can write a letter that better captures who you are.
Biochemistry & Economics
If you are applying to schools through multiple application centers (AMCAS, AACOMAS, and TMDSAS) Interfolio allows your letters to be submitted only once: to Interfolio, and then you can direct the letters to each application center you are using.
Most letter writers are comfortable with using multiple application centers, since the process is relatively simple, but many applicants still choose to use Interfolio for the ease of it (though it is a paid service)