You only need to save the email notifications you get automatically from Italki after each of your credit payments and again after verifying a completed lesson. I do not know about other countries but emails are now accepted as legal proof of receiving or paying something at least in the UK there was a time more than ten years ago where sometimes a paper copy and a separate later 'verifying' signature may have been requested for official and legal situations, but not any more.

Your Email notifications are proof of payment and proof of the teacher /tutor receiving the payment.

No need to risk inviting unscrupulous people.

Hi Marcos, I could give you a receipt after each lesson.

Kimberly, I need to do about about 4 lessons per week. We can do here in Italk, but I need the invoice. Could you help me? I can do the lessons AT 8pm Brazilian time zone

Hi Kimberly, how can we talk in private? Do you know?