As Charles mentioned, the Painted Accent Stripes were a "Stand Alone" Option on 1966 Mustangs. This similar Option was also available on 1967 and 1968 Mustangs, at the same $13.90 price tag.
I believe that in the "scallops" you mentioned, you are referring to what was the "Standard" GT Stripe for 1968 (see picture attached). Unlike the painted Accent Stripe, this "C Stripe", as it is known by, was an adhesive transfer stripe. It was available in several colors ; Black, Blue, Red, White and Gold.
Interestingly, if you didn't like the new "C Stripe", you could order, at no additional cost, a tri-bar GT Stripe similar to those used on 1965-67 GT's.
Hope this helps clarify things for you.
Bob