He has bought a banana case.
“Un casco di banane” is more like a bunch of bananas. Treccani has it as a fancy botanical term “infructescence” a.k.a an aggregate fruit. I truly don’t know what I would get from the store if I had ordered to buy a “banana case”. Probably a huge case of bananas? Or, I think I got it : “un casco” is a “helmet” so, a bunch of bananas looks like a helmet. Yes, “un casco di banane” has a much smaller size. I would get that!
Hi. Yes, I’ve heard this use of casco during various trips to the supermercato. Very descriptive.
Uhm… Actually, for “casco” indicates the whole infructescence (like the linked image), that may have hundreds of individual bananas. At the market, bananas are normally sold in small clusters of 4-6 bananas. I wouldn’t use “casco” for that… I would call that simply “delle banane”.
I don’t know if English has a term for that. I would have translated the English sentence as “una cassa di banane”.
Friend in Lucca had a great sense of humour when shopping, I probably misheard when it came to le banane;-)