I agree with some of the others, most health issues in fish are triggered by stress (such as chasing, netting, bagging and moving a fish to a new environment), and as boldtogether stated if your water parameters are off from what they were housed in at the LFS, that too could cause a sensitive fish to go belly up. This is why a good vendor won't sell newly shipped/imported fish right away - there will almost always be some mortalities in every shipment, some just take longer to weaken and die than others. So if the shop had them for 1-2 weeks, chances are whatever they were doing, didn't kill the fish. That seems like a reasonable conclusion to me, but short of a necropsy no way to tell with any real certainty. If they give you half back in store credit I think that would be more than fair.
Not saying this was your fault, but I don't see how any business that gains a good portion of their revenue on the sale of live fish can survive if they refunded every fish that died while under a new owners care. Big box stores can do it because their live fish sales play such a small part in their overhead/sales, with cat/dog and fish dry good items being the bulk of their sales.
Also, overpaying is relative to the situation. When one buys locally they actually get to view the item, before purchasing. To me that is always worth paying the extra for, vs buying an item sight unseen.
Good luck