I've noticed that some of the content locked behind the Ubistore still shows up in the customization area. If you go under backpacks and see that there's one from the "Yeti Hunter" pack, it may have the option for you to see its alternate color schemes. If there is no YETI hunter backpack, then the entire outfit is a costume much like the Dying Earth costume from the U-play rewards area. Costumes cannot be customized nor can their individual pieces of attire be accessed. I hope that helps, sorry if that's the case. On a side note, that shotgun looks pretty sweet.