Your best bet is to get them directly from ICE anyways - http://shop.ironcrown.com
Especially since we currently have all of our products on sale for 25% off their regular price. DriveThru apparently isn't sorting products by sub-categories.
As for determining what products go with what version of Rolemaster, might I suggest taking a look at our main website (you can do that by click on the crown in the upper left corner). On the left side of the page, there is a heading that says "ICE Games" and beneath it is a listing of the major lines.
Clicking on one takes you to a page that gives a brief description of the main products for that line (for example --> http://www.ironcrown.com/index.php?page=igames/IntroRMC <--), and a list of "Compatible Products" underneath. Clicking on the name of one of the compatible products takes you to a page that describes it. For example, clicking on "Express Additions" will take you to a page that gives the table of contents for all of the issues of Express Additions.
And by looking there, you will see that if you have the core RMC books, that many of the first issues of EA you won't need unless you want some of the optional rules in them.
I hope that this has been helpful. Enjoy!