Learn Italian: Prepari la fattura e mi dia il rimborso. - Prepare the invoice and give me the refund.

Because this is a formal command (using the polite Lei form). With informal commands, pronouns attach to the end of the verb (like dammi for 'give me'). But with formal commands, the pronoun must go before the verb.

Doesn't prepari usually mean 'you prepare' in the informal tu form?

Yes! In the regular present tense, tu prepari means 'you prepare'. However, for commands (the imperative mood), the endings swap. The informal command is prepara ('prepare!'), while the formal polite command is prepari. You can tell this is a command because it is paired with another formal command, dia.

Why does the formal command for dare end in -a (dia) instead of -i?

Regular -are verbs like preparare take an -i ending for formal commands, which is why we see prepari. However, dare (to give) is a very irregular verb. Even though it is an -are verb, its formal command is dia, which is simply an irregular form you have to memorize.