Hi Patricia,
The only way I can think of this working is to have a whole bunch of custom fields (up to a pre-defined maximum number of different products per form).
e.g.
Item 1: (select from drop down) Item 1 Qty: (enter number)
Item 2: (select from drop down) Item 2 Qty: (enter number)
Item 3: (select from drop down) Item 3 Qty: (enter number)
Item 4: (select from drop down) Item 4 Qty: (enter number)
Item 5: (select from drop down) Item 5 Qty: (enter number)
If you wanted to, you could sum the total quantity of items using a calculated field.
Not sure if this helps, but I'm interested to see if there are any other solutions out there.
I guess you could alternatively submit 1 form per type of item (i.e. just select the item type and enter the quantity), but then you would need to also enter some kind of overall order number on each form to tie them together.
Regards, David