I'm not quite sure how to answer you since I don't know what "kind" of products will be in the store, but I'm thinking something very very super basic would be all you need:
any other fields you require for their ordering
Item 1: Qty:
Item 2: Qty:
Item 3: Qty:
and so on for however many you feel are needed
text boxes of course would be to the right of each Item #: and Qty: so they can just fill them in themselves.
One way to maybe try to make this easy is to make the Order page outside of the cart so it will open in a new page using the target="_blank" attribute. This way they can have the order page open and fill it out as they browse the catalog.
Make sure you show them how it's done on the home page or somewhere easy maybe a page in the shop called "Use our Pop Out Form" or something like that which I am sure needs to be shorter. Give good instructions so they know to open the form page and keep it open while they order so it makes it easy.
Have them fill in the item numbers themselves to keep it simple, but if you feel energetic you can always make dropdowns with all the items in it. You can create custom lists and reuse them too so you wouldn't have to retype it for each box.
Lots of ways to do it, unfortunately I have none up that can help with showing it, but an example of what can be done with the dropdowns you can see at the CatTown site in my sig. The contact page has a contact list dropdown, but that can be whatever you want to put in there so you could make a dropdown for each line instead of empty boxes. I personally think if the item numbers are short enough that having them fill them in would be faster. Tell them instructions for copy and paste and that works great too then.
Hope that helps a little at least.