There have been stories for years of mail order (and now online) retailers that pull a scam that goes something like this:

• You respond to an ad for an insanely low price on a lens. They accept your order and bill your credit card.
• A few days later they contact you to say the exact lens is out of stock, but they will sell you the upgraded version for only $$ more.
• If you tell them you will wait on the item you originally ordered they will never send you anything (mainly because that version does not exist at all). They will try to string you along until it is too late to reverse the charge on your credit card.
• If you agree to the upgrade, you wind up paying more than you would have if you had bought the lens from a reputable dealer to begin with.

To the best of my knowledge, Canon has never made the same lens with both plastic and steel mounts. They have never released an "L" lens with a plastic mount. The only Canon lenses I've ever seen with plastic mounts were kit lenses, including the EF 50mm f/1.8 that was once available as a kit lens with early EOS film bodies. The EF-S 18-55mm f/3.5-5.6 has a plastic mount for most if not all model variants, as did the old EF 28-80mm and 28-90mm lenses that came with a lot of film Rebels. The EF-S 55-250mm variants all also have plastic mounts.