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A “plug” is where you advertise something. People will often go on TV shows to “plug” their latest book.
Cheap is probably meant in the sense of “cheap shot”, as in easy, poor taste etc.. but I wouldn’t say it’s common to use it like that.
My guess is they meant you had blatantly advertised something where they thought you shouldn’t.
They meant you advertised your business without paying a fee for the priviledge of doing so. A plug means to advertise a business or product.