I can't figure out how to list a product with a value-added feature(s) when the "plain" product's UPC is already listed on Amazon.

For instance, if I want to sell a hydration bottle with an added shoulder strap that doesn't have a UPC, but the plain bottle is already listed, I don't know how to list my product with the added strap, because when I enter the UPC for the bottle, Amazon insists that use the existing listing.

Or maybe I want to offer a product in a storage case or box, again with no UPC of its own, which the existing listing for the "plain" product without the case doesn't have.

Variations don't seem to be the answer, because Amazon says that all variations require their own UPCs, and of course adding a feature like this doesn't change the base product's UPC.

Bundles don't seem to be the answer, because Amazon says that all items in a bundle require their own UPCs, and "added parts" don't always have UPCs.

I've also got a product that comes from the manufacturer in one of two random color schemes. The existing Amazon listing doesn't depict or discuss this, showing only a single photo with one of the colors, so I want to make a new listing showing the possible color variations, along with additional text explaining that the product will arrive to the customer in one of two random color schemes. The UPCs on the products are identical, though. This same product is like the ones I as about above, as well; I have a value-added feature that I supply with the product, and of course the original listing doesn't have my customization.

Any help here is appreciated.