Hi Bosskh -
Having done this a few times at FRA since 1994, I would suggest the following:
1. If your chosen LH case, or cases are paid for before you arrive, I would make your pick-up upon arrival, and not departure.
One and one half hours is more than enough time to accomplish this, even for a traveler new to FRA. It will also allow you a little time to browse in the LH Worldshop for other purchases.
Waiting until your departure allows more risk into the purchase picture at the end of your trip. Your flight arrival can be late, you become delayed, risk of a strike, etc. There are many possibilities to consider, so why not make the effort upon arrival ?.
But coming into FRA, with your items pre-paid, all you must do is go over the FRA map, or pay the small token amount to have someone meet you from LH at the gate with your new luggage case. The LH staff can certainly escort you back to the LH Worldshop in short order for more buying, which I'm confident they would be happy to do !
Keep us posted.