I haven't heard of Pakistan issuing a stamp on plastic, and Scott and Gibbons don't state that the stamp is printed on anything other than paper. The design is Scott #510, however.
Countries are issuing stamps in different formats. I listed some of the different styles along with a few country names as follows (off the top of my head): holograms (US, Canada), scented paper (Korea), stamps printed on wood (Paraguay, Switzerland), stamps printed on polyurethane (Austria), embroidered stamps (Switzerland, Austria), and it looks like simple plastic (Pakistan but not sure), lenticular plastic for 3-D effect (sand dune countries, Bhutan), plastic records (Bhutan), CD ROM discs (Bhutan), various metals like aluminum and silver (Russia, Netherlands, Bhutan, Tonga, sand dunes, Qatar, Jordan), wood chips, powdered rocks (Hong Kong, Guernsey). Canada issued some hockey souvenir sheets where the sports image moves like a movie. As technology advances, we'll see plenty more stamps like this.
I edited this to add a few more items and countries.
Michael, that is fascinating. I had no idea so many different materials were used for stamps! It would probably make a great exhibit for SOR and elsewhere!
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It's kind of like a puffy sticker, but it's not that soft. I can press it down, but I have to put a bit of finger pressure on it. Thanks for the responses.
I think you have a stamp that has been coated or dipped in Decoupage epoxy. That is a
two-part epoxy used to create a collage of decorative objects. I once knew a guy who
took cigar boxes covered them with stamps and then sealed with Decoupage epoxy and
sold them on Etsy.com to stamp collectors, as gifts.
From what I can tell it was cancelled on the plastic material. See picture. The cancellation ink is affected by the plastic material. I took this with a USB microscope.
One more thing after more carefully examining it is that the plastic is slightly thicker on the right side of the stamp. There is also plastic material joining two of the perfs together on the bottom right. Also there are 2 spots where the shiny material is on the back. It is dull otherwise on the back.