Invoices are generated automatically with each top up of the wallet. When you make a top up you must choose the amount to be recharged and the payment method, which can be a credit or debit card, or via Paypal. According to all the billing data indicated in the profile, an invoice will be generated and you will be able to download it in the wallet's history.  Therefore, if you want to generate invoices for several customers, you will have to modify the billing data for each customer each time you top up. Unfortunately, it is not possible to save multiple billing data but this is a feature that will be available in the future.