Rude gestures are rare on postage stamps, but Ukraine's best known stamp has one. It shows a soldier raising the middle finger to a Russian warship in reference to a stand-off at Snake Island on day one of the full-scale invasion nearly three years ago.The Russians demanded surrender but the Ukrainians refused, using unprintable language.The warship in question, the cruiser Moskva, was sunk by the Ukrainians two days after the stamp was issued, and it sold out within a week of going on sale.Such is the significance of the stamp that whatever was left was given to government delegations representing Ukraine on the world stage.Ihor Smilyansky, the head of Ukraine's postal company Ukrposhta, acknowledges it was a risqué step to take."It was my decision. I said - I don't care whatever everyone else thinks. I just believe it's the right thing to do," he told the BBC. "I know it's breaking all the philatelic [study of stamps] rules and all the rules. But we're about breaking the rules."Ukrposhta often tests its designs on the public, and the results of such online polls tend to be very political too.That was how Ukraine's best-selling stamp came into being, showing a Ukrainian tractor towing a captured Russian tank and featuring the popular wartime greeting: "Good evening, we're from Ukraine."Ukrposhta has sold about eight million such stamps.Stamps featuring Ukraine's famous mine-sniffing dog Patron earned Ukrposhta about $500,000 (£400,000): 80% of the money was spent on mine-clearing equipment, and the rest on animal shelters.Another stamp of a mural left by renowned graffiti artist Banksy on a building devastated by shelling outside Kyiv, helped fund 10 bomb shelters. This stamp features another popular but unprintable Ukrainian slogan - this time directed against Vladimir Putin.Ihor Smilyansky says a dose of humour is added to Ukrposhta's stamps to maintain Ukrainian morale during the war with Russia."Humour has become a fighting force for Ukrainians in this war," he tells the BBC. "Even in the most difficult circumstances you have to take it with a sense of humour. And that's what our stamps are sometimes about."Oscar Young from UK-based stamp dealers and auctioneers Stanley Gibbons says Ukraine's app ...