What kind of pen works on the DCL souvenir luggage tags?

[Cheburashka]

I've used permanent marker, but it very quickly becomes extremely faint. Has anyone found a type of pen that shows up well and doesn't quickly fade away on DCL's souvenir luggage tags?

 

[dmesich]

I've used permanent marker, but it very quickly becomes extremely faint. Has anyone found a type of pen that shows up well and doesn't quickly fade away on DCL's souvenir luggage tags?

I use mine as coasters.