Note that the Ukrainian postal service managed to unveil a stamp commemorating destruction of the bridge within hours of the explosion.
I'm assuming Photoshopping up a taunting stamp design is easy and fun when you're patriotically inclined.
(stolen from someone's .signature)
Zelensky is a former actor who clearly understands PR, so it is not unlikely that he would plan something of that sort.
It doesn’t seem like it would take hours to grab a screenshot off of a TV broadcast and photoshop it onto a stamp, without advanced knowledge.
The screenshot-the-TV approach also works, and there are ML filters that can stylize that art, so we wouldn't know how the stamps were actually created.
I expect the Ukranian post office has many similar stamps designed and ready. They make good material to promote Ukraine.
We'll know it was the Ukrainian postal service when someone asks Putin to sign to confirm delivery of a whole parcel of humiliation.