I don't think you can add the Extra Pack I and Extra Pack II downloads to add to the boxed version. Just Flight package it differently to the way Captain Sim do.

At the online store, the C-130 X-perience is €39.99. The Just Flight boxed version is €44.95 + Packaging & Handling (postage). So you could have got it cheaper buying the download version from the CS online store.
However! If you did not know that, then it's just a pity that you didn't.
My problem is that when I buy something, I want it now. Not a week later via post. I have done that, I still do, but I get so impatient for my postie to deliver it.