Hey there again! 
Is the iridescent part metallic? It should be, if it isn’t. Your specular highlight doesn’t look right which is why I ask.
Yeah, the light scene makes it a little hard to see (the skylight is using an overcast HDRI to get a somewhat neutral lighting, which doesn’t exactly help in this case). The foil part has full metallicness and with the skylight turned off (and only the directional light remaining) it looks a lot more realistic on that front:
This effect is a bit annoying to do with a roughness/metallic setup (like Unreal uses) because we can only change the specular color by setting the metalness to 1 and adjusting the base color. You can use the clearcoat shader if these cards normally have an additional clear coat over them (I imagine they must? Or they’d be pretty delicate? Not sure)
Hmmm, good question. I actually don’t think there’s a clear coat there but there might still be a way to utilize the shading model for this in some way. I’ve messed around a little with it recently and I think there might be an application somewhere out there for this!
Another option would be to just create a cubemap with the highlights in them and sample it from the reflection vector of the cards surface blending it over the existing iridescent effect might be convincing enough.
Honestly, that doesn’t sound too bad at all! I haven’t considered using a cubemap yet but that’s a good approach! 
I’ll see what I can do utilizing that!
You can use a stencil mask or alternatively you can just use a transparent plane that hovers above the region you want the post process applied to. It’s used a lot to give a post process effect that only appears behind the plane (depth fog for water is usually the most common example.) Both of these will require you to recreate the chromatic aberration effect though
Hmmm, I see. Yeah, I’ll try that after the cubemap.
Thanks for the help so far! 