Learn French: La facture ci-jointe confirme le montant à déduire. - The attached invoice confirms the amount to deduct.

Because it comes after the noun it describes, la facture. When words like ci-joint (attached) or ci-inclus (enclosed) follow a noun, they act like normal adjectives and must agree in gender and number. Since facture is feminine singular, it becomes ci-jointe.

Would ci-joint still have an -e if we wrote Vous trouverez ci-joint la facture?

No, it would not. When ci-joint is placed immediately before the noun it modifies, it acts as an invariable adverb and does not change. It only agrees with the noun when placed directly after it, as in our sentence la facture ci-jointe.

Why do we use à in le montant à déduire instead of just the infinitive or pour?

When you want to say a noun is to be acted upon (like an amount to be deducted, or a form to be filled out), French uses à followed by the infinitive. Using pour here would incorrectly sound like the amount itself has the purpose of deducting something.