Customize your card using fluorescent spray paint our range  Pintyplus Basic Fluorine !
Group the cards and fires away at short intervals on the profile of the package. It should be a very subtle work to prevent the paint from seeping between the cards. Apply the paint by far for the solvent to evaporate as much as possible before actually touching the surface.
In the link below you’ll see how easily achieve this effect;)
Source: Behance