Oversized Luggage Penalties

I’m about to buy a new set of luggage, from a store here in Montreal. I just wanted to double-check if I was getting a good price, which I am. However, I see the following at Suitcase.com:

First of all, what do they mean by “linear inches”, since this particular bag is a 30" pullman? Length plus width plus depth?

Secondly, every airline must be different and I’m not about to call them all up. Does anybody know if this “caveat emptor” is a genuine concern or just the company covering their behinds, in case of a later problem? (Or maybe making you buy two smaller bags instead of one, therefore more profit?)

Airlines are getting “extra charge” mad lately. Over weight, over size bags are routinely being charged extra even for domestic flights in the us. AA for one charges $70 for over-weight, over size bags domestically. For more info go to flyertalk.com and do a search for your particular airlines of choice.

And you are correct the linear inch thing is W+T+D. I am buying a rolling duffle and will make sure it will not be oversize.