Just asked a Staples manager same thing yesterday, he said "Yes unless the code says for internet use only" Just looked @ my last Staples postcard & it says Valid through Staples.com only. I have had cashiers ring them up for me though.
A friend of mine warned me about my pricematching @ Staples lately & suggested they'll have a photo of me from their security cam up in the employee breakroom soon, with the title, "Watch out for this guy"