Rule reminder
When transforming from informal to formal Italian: (1) replace 'tu' with 'Lei', (2) replace colloquial vocabulary with formal equivalents, (3) use passive or impersonal constructions where possible, (4) eliminate fillers, (5) use formal epistolary formulas, (6) prefer longer, more precise expressions over contractions.
Informal: 'Hai sbagliato a mandarmi questa fattura.' — Best formal complaint version?