Learn Italian: L'autobiografia è un trionfo per aver venduto tanto. - The autobiography is a triumph for having sold a lot.

We use per + past infinitive to express the cause of something that already happened ("for having sold"). If you used the present infinitive (per vendere), it would express purpose ("in order to sell"), which changes the meaning completely.

Why is the helper verb written as "aver" instead of "avere"?

In Italian, it is very common to drop the final -e of an infinitive when it forms a past infinitive (like aver venduto or esser andato). Avere venduto is also correct, but aver flows much better in spoken Italian and is widely used.

Could I say "perché ha venduto tanto" instead?

Yes! Perché ha venduto tanto (because it sold a lot) is the explicit way to say this, using a conjugated verb. The per + past infinitive structure is an implicit alternative you can use whenever the subject of both actions is the exact same (here, the autobiography).

Why doesn't "tanto" change to "tanta" to match the feminine noun "autobiografia"?

In this sentence, tanto is acting as an adverb meaning "a lot," and it modifies the verb venduto. Adverbs are invariable, so they never change their endings. It would only change to tanta if it were an adjective modifying a feminine noun, like in tanta fama (a lot of fame).