It's shady stuff like that that gives lie to the notion that one should always support B&M retailers. Support good B&M retailers, sure, but not crap like that. I probably would've done what you did. Well, I probably would've been more of a dick about it. Upon seeing the slip, I would've, without saying why, instructed the salesperson to fetch his/her manager. Then I would've explained to the manager what just transpired. Finally, I would give the manage the option of giving me the internet price ($380) to make up for this blatant attempt to cheat me, or to void the transaction and leave a bitter taste in my mouth. If s/he chose the latter, I might have asked if they had a computer I could use for a minute.