does dollar tree sell knock offs?

Manufacturers do specific product runs for dollar store retailers that they don’t sell anywhere else, like the 20oz LaCroix. It isn’t a knock off, it’s a product specifically packaged for that outlet. Do you ever go down the candy aisle at the dollar store? There are a ton of types of candy in the theater box format available there, types that you can’t find in that format anywhere else (including the movie theaters). The same thing is done for club stores, like Costco or Sams Club. Products are packaged in a format specifically for those retailers based on what they want (in the case of club stores it is generally larger quantities and/or product mixes in the same package, like four different flavors of granola bars boxed together in a big box). No scam or questionable practices involved.

They buy close outs form stores that are going out of business.  They can buy their stock at 10% of what it would cost new.

Look at the expiration dates BEFORE you buy.  Expired lotions, shampoos, soaps etc can give bad allergic reactions.

Sometimes the manufacturer makes special runs for discount chains. It is probably the same thing, or with minor changes. 

I used to get a particular shampoo at the dollar store that had less fragrance. It was the only place that I could get it.

They buy in huge bulk lots. Some items may be close to expired though,  check dates.

They are too big to risk selling knock offs and getting sued by the likes of Dial and LaCroix.