After re-reading your post, I see you have a Silverado, so a lot of what follows doesn't apply. I thought you did not already have sissy bar, etc. (duh)
You may have been looking at fender racks rather than the oem "rear luggage rack". Fender goes on fender; luggage fits behind sissy bar. Part STR-4WM51-20-00 is the rear luggage rack, but you would also need the side rails and sissy bar (short or tall) plus backrest. Getting into an expensive proposition just to add the luggage rack.
If I were going to add all that stuff (my silverado came with all but the luggage rack), I might consider the new "quick release" parts that are now offered. They cost plenty, but none of the older stuff will fit the QR parts per Yamaha. The
main reason for this would be allowing you to modify the look of your bike for two-up, touring or single riding much easier.
Backrest Sidearms - Quick Release STR-5VN41-10-00 $246
Backrest upright - Quick Release Short STR-5VN41-30-01 $115
Backrest upright - Quick Release Tall STR-5VN41-30-02 $125
Quick Release rear luggage rack TBA - may be available $$??
BTW, the QR side arms also come with eyebrows. I think these parts may be cheaper than comparable older parts that are not QR compatable.