The price shown is always including VAT
This is not true. Prices usually are VAT excluded. As is customary on a US based website.
https://itch.io/docs/creators/payments#vat-support
Calculate the VAT rate to collect based on the buyer’s location, and optionally bill it on top of the base item price (exclusive). This option can be disabled to have inclusive VAT pricing.
Did you activate vat inclusive pricing? That is nice of you, I guess. If only you could ensure all your competitors would do the same.
But till Itch implements features to adress special vat rates, the key issues is as I explained: your customers are not buying your book, they buy access to your project page's downloadabe files. If you upload example programs to go along with your programming book, then what? How would that affect taxes?
People expect prices with VAT included
People in the EU shopping on a EU site do that, because it is the law. People shopping on a US site and know that they shop in the US, do not.