They are a digital store, so it is normal that they have MOSS.
Personally, I think they mean that they allow you to sell rewards, not that they are a physical store that sells rewards. In other words, it is up to you to deal with the hassle of import taxes.
In all the years I've been on Itch and its forum, this is the first time I've seen this question and it doesn't seem strange to me that they don't have the document you're requesting, but I don't work at Itch.
I wish you luck, I hope they have the document you need and the staff responds to you, but if I were you, I would look for a plan B in case Itch does not have an IOSS.