Would simply clicking a row in the products table to add it to line items be acceptable?
Drag and drop will copy the contents of a specific field to the field into which you "drop" the dragged contents. That could be made to work and without any scripting, but only if you drag from a Product ID field in one portal to a product ID field in the other. On the other hand, you can set up all the fields in the portal row as a button such that just clicking the portal row performs a script that uses set variable/set field to copy over the same product ID.
I currently have it set up as a button with script, which to be honest works fine for me. however i am trying to match how an older piece of software works as i think it will be more familiar with the staff that will end up using the solution.
I think it may be easier to stay with this option.
Thanks for the reply