I get the same thing when I view the individual products, but once I go to the cart, it says that I am not eligible for 20MUSA. Is there an entry path of some sort from RS?
Edit: Same thing reported here
https://rokslide.com/forums/threads/midway-usa-promo-codes.398061/post-4082321
Couldn't find the 'promotion rules' alluded to in the cart message, so I called. Turns out this code was emailed to certain customers who had "not made a purchase on their accounts in some time" and only works from those logins.