Well, as it turned out, I did have Velma's collar - it had been put on Shaggy from the same set. Shaggy is missing his collar, so I changed my order at DS & hopefully it will all work out.

EDIT Even sillier: I looked at my pics from August 2020 & saw that Velma was collarless & Shaggy was wearing Velma's collar. I dug through my tub of Scooby series 1 & there was Shaggy's collar. It had gone unnoticed the entire time. DUH.