that we bought our puppy from petland as well....if only we knew then what we know now but then we wouldn't have our miko. we certainly won't make the same mistake twice though.
it sounded from your last post like the original vet is diagnosing liver shunt for sure, lots of smaller ones. is that right? if that's the case, i would get a second opinion from a vet not affiliated with petland. if you get the same diagnosis a second time and the petland vet still won't diagnose it (if they say it's vaccination related then it's not genetic, they won't have to replace your pup if you lose him) then with the two diagnoses from 2 separate vets, that would certainly put pressure on them.
if worse comes to worst, tell them you have a family member who's a lawyer who has offered to look into your contract and they're prepared to make petland abide by the contract.
please keep us posted. of course, after all i typed above, i'd much rather hear from you that your fitz is feeling better and is on the mend.
please take good care.
my heart breaks for you and your pup.