Tell you what. If the rear u-channel was still available from GM (this is the one that seals the rear window to the topper) I would pay double what LMC charges. I don't like throwing money away, but I know that the GM seal would be made properly and be durable.
Skimp on price, almost a surety that you will get screwed in the end. Now with that said, more and more weatherstrip is being disco'd from GM, so buy it now while you can.