The FRA terminal maps are a little hard to read, but make it clear the LH WorldShops (there are 2 of them) are landside. The WorldShop map for one of them is pretty good:
https://www.worldshop.eu/page/positionPlanFrankfurt?4
[I posted the link alone because for some reason my browser doesn't always connect to the map. If you search 'Frankfurt' on the WorldShop site, position plan will open the map]
The other WorldShop is in departure area A, but I haven't stopped at that one and WorldShop hasn't posted its map yet.
Not sure what your itinerary is. but the WorldShop can hold your purchase and if on your return from Geneva you don't want to exit airside to pick it up, can deliver it to you at your connecting gate.
I am not sure how that works for a bag like a Topas which when boxed, is too big for carry-on. Maybe your airline can gate-check it for you.
In any case, check out this thread. I was in FRA 2 weeks ago and in post #192 priced the Topas I think you are looking at:
Discussion re: Rimowa prices
If you search topics here you can find a number of threads re: buying Rimowa in airports and the related VAT questions.