I bought a customization for all the items I usually wear.
When I wanted to change one of the customized items, I realized that it did not have the option yet - so I bought it (again) - only realizing that it should be customizable already after I went into the customization menu.

Using the Steam standalone version of the game on Windows 7.

Which item, for reference?

Erm, yes - sorry, it's the Bell Scarf

I'll try to test on an alt in the next few days if it's an issue strictly with the item. Some items glitch when attempting to customize them for some reason (the Pokeball ball shaman item when it released, for example).