In a nutshell: Unless you have a committee packet, you will need to have each letter uploaded to Interfolio separately. That being said, you can create your own "packet" when you enter the school you want to send your letters to. Technically you are sending them as individual letters, but since they are part of the same "delivery order" they will all be sent together, as they would in a single packet.
This topic comes up every year, so I suggest you do a search of the forum, particularly in this thread:
http://forums.studentdoctor.net/showthread.php?t=915502, which is the Official LOR Thread. In fact, this thread will likely be moved by the mods to that thread. Below are instructions for sending letters as a "packet" within the same delivery order. You may already know how to do this, but at least by listing it here it will clear up any questions others might have.
Instructions:
1. When you log into Interfolio, go to the Home screen.
2. Click on "Start Delivery" tab. You will be directed to the "Add Delivery" page
3. On the Add Delivery page, begin entering the school to which you would like the letters sent.
4. Find your school and click on it.
5. Once you have selected the school, click on the "Add Documents" tab.
6. Once you have clicked on the "Add Documents" tab, you will be directed to the "Select and Order Documents" page.
7. Select which letters you would like to place in this order from the "Documents available for delivery" list. They will appear to the right of the list of your uploaded letters under "Documents in this delivery" Note: This "order" can be 1 letter or all of your letters.
8. Once you have all the letters you would like to send to a particular school/campus, you will need to click on the "Customize Delivery" tab below the "Documents available for delivery" list.
9. At the "Customize Delivery" page, make sure your AACOMAS ID is visible and selected (the correct number, of course- if you used Interfolio has a middle-man for AAMC, make sure your AAMC ID is NOT the one selected). If your AACOMAS ID is not visible, click on the "Add ID numbers" and add your AACOMAS ID.
10. Once the ID is selected, click on "Choose Shipping Method" which will allow you to review your order (make sure all the documents you wish to send are listed under "Documents" to the right.
11. Choose shipping method and click "Check Out"
12. Proceed to Check Out and finish.