The Invoices page lists all invoices generated as a result of an order.

With the two buttons at the ID column, you can

download the invoice
do a refund of the invoice amount

If the global configuration is set to store the invoice locally the invoice can be downloaded as it is. If the invoice is not present, a new invoice will be generated before download.

Handling Pay Later

If enabled, a customer can choose the pay later payment option. So the order will be completed and the customer gets an invoice mail. After the customer has paid, you have to process the payment and final the order. An open order will show a payment button. To apply a payment, click the button and enter the amount paid. The open amount is set by default, but the customer may have paid to less. If the amount to pay is filled up, the payment button will be replaced with a process button. Click this button to process the order completely. The order processing will generate calculate the period to access the downloads and, for new customers, unblocks the Joomla! user account.