In the UK, that would have been a large letter. Standard letter – no thicker than 5mm (and C5 – the envelope for flat A5 paper). Large letters go up to 25mm thick, and whatever the envelope is to hold A4 flat.
On the thickness front, so annoyed that there are many more choices of puffy stickers than flat in stationery/craft stores…
And although the UK doesn’t do a square non-machinable surcharge thing, I’m also a little disappointed so many cards are square (and perhaps fall foul of minimum sizes for international mail).