I don't think this is malicious, registrars often have to use third party top domains like for example .tech which reserves common words as premium domains, but you are unable to see the price until you make a request for buying it. So in this case, Namecheap would show the default price, but when it tries to process it, they would receive the premium domain price - thus, since it's no long matching the cart price, they have to update it to reflect the actual price.
I'm sure they could do this more informatively though, but they're not alone in not doing that..
I'm sure they could do this more informatively though, but they're not alone in not doing that..