poŝtkarto

Two roots, no seam to worry about. Poŝto plus karto, glued directly with the first root losing its -o. That elision is optional in Esperanto and here it is the ordinary choice, exactly as poŝtmarko (a stamp) and poŝtoficejo (a post office) do it. If the join ever looked hard to read you could write poŝtokarto and nobody would blink, but nobody does.

The compound is transparent and that is the point. German has Postkarte, French has carte postale and English has postcard; Esperanto did not have to choose between them because it could build its own out of pieces the learner already owns. This is what people mean when they say the vocabulary is smaller than it looks.

Why this word is more than a souvenir. Before the internet, the Esperanto movement ran on paper: you found an address in a yearbook, you wrote to a stranger in another country, and korespondi was not a hobby word but the main way most speakers ever used the language. Postcards were the cheap end of that traffic, and the illustrated ones with a greeting in Esperanto on the back are now collected as movement history. The habit survives in the annual card exchanges and in Pasporta Servo, which is the same instinct with a spare bed attached.