As ibob52's link shows, it is just for Prime members. It also said on my page that it is only for first-time reload using a credit or debit card. If you have reloaded your account this way before, I guess it wouldn't apply to you.
From the site - In order to qualify, you must (1) be an
Amazon Prime member; (2) this must be the first time reloading your Amazon com Gift Card Balance and the reload amount must be at least $100; and (3) you must use the Amazon Prime account to which you are currently logged in.