I put this to the test recently when visiting the site of a leading sporting-goods purveyor in search of some camping gear for the summer. The gear was readily available. Free shipping wasn't.
So I went to Retailmenot.com, where I entered the website of the sporting-goods company and came up with a recent discount code that other people have used for that business.
I applied the code to my own purchase and -- bingo! -- all shipping charges and taxes were erased from my bill, representing about $30 in savings.
This is probably wrong for all sorts of reasons, not least taking advantage of a discount that was offered to someone else and denying government authorities a few bucks in tax money.