Your passport. If you hold an EU passport, you should also have a document proving that you reside outside of the EU: consulate card, certificate of registration in the register of French citizens established outside of France, « settled status » or « presettled status » document for residents of the UK, green card or any other residence permit from a non-EU country.
All items listed on your tax-free form. In theory, they should not have been used. And it is advisable, although not formally required, to present them in their original packaging. In case of control, the absence of a single item listed on the tax-free form or the failure to convince the customs agent that the items have remained unused can lead to the cancellation of the entire form.
Your tax-free forms (digital or paper) and accompanying invoices issued by ZappTax at your name and which proves that ZappTax resold the items to you.
Your flight information (ticket, reservation, etc.).
See also question: