Yes, I know this. Tried to reason that with them but they just seemed frustrated with me and continued to refuse. I'd find another LFS if getting there through Atlanta traffic wasn't a literal fight to the death lol1 - Rock can be easily broken with a hammer
2 - it only takes a half cup (or less) of sand
I would find another LFS
Or simply a local reef club, or another local reefer to grab some sand or rubble from.
Edit: see all of this already mentioned.