Learn Italian: L'importo dell'imposta è troppo alto. - The amount of the tax is too high.

The adjective alto (high) is describing l'importo (the amount), which is a masculine noun. Even though dell'imposta (of the tax) is right next to it, the core subject of the sentence is the amount, so the adjective must agree with that masculine subject.

Is importo a false friend? Does it mean "import"?

It can relate to importing goods in specific contexts, but in financial and everyday Italian, l'importo simply means "the amount" or "the sum" of money.

I've also learned the word tassa for "tax". What is the difference between tassa and imposta?

In casual conversation they are often used interchangeably. Technically, an imposta is a general tax based on your income or wealth (like income tax), while a tassa is a fee you pay for a specific public service, like garbage collection or a university enrollment fee.

Why is it dell'imposta and not just di imposta?

In Italian, when a noun takes a definite article, the preposition di (of) almost always merges with it. Here, di combines with l' (the article for the noun imposta) to form the single articulated preposition dell'.