I don't want to put words in your mouth, so can you clarify that this is what you're saying:

(1) Sam's fulfilled your prescription.

(2) It wasn't covered by insurance.

(3) Sam's was not the insurance carrier and the reason that the insurance didn't cover wasn't Sam's fault.

(4) You paid, on your own free will.

(5) Your doctor gave you one for free.

(6) Because you found a cheaper way to get this item, you want Sam's to take it back.

(7) The only rational you have for them taking it back is that they make a good profit.

Is that pretty much it? They are a profitable company, so they should accept back an item that you agreed to pay for.