Are you suggesting the Ukrainian postal service pulled it off? This was months in the making; I bet they already have projects for the liberation of Crimea ready.
(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.
I expect the Ukranian post office has many similar stamps designed and ready. They make good material to promote Ukraine.
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.
We'll know it was the Ukrainian postal service when someone asks Putin to sign to confirm delivery of a whole parcel of humiliation.