The top-up history tab allows you to view the history of your top-up payment, with the specific data and time, transaction code, response status, whether it was approved or failed, amount credited, and the invoice for the payment. Only a top-up with a response status of Success will be credited to your account.
To view top-up history and invoice hover over the Configuration icon from the Navigation bar and click Billing. Top-up history and Invoice is the the the fourth item in the menu bar.
Clicking Show Invoice will display a 'printable version' option for your invoice.