Each deposit generates an email receipt. If you didn't happen to save that email, you can view a record of your deposit by visiting the "accounts" section of our site, selecting the account in question, and then visiting the "Search Account Activity" link in the "Actions" box. (Note that if you made the deposit within the last 12-24 hours, it might be displayed near the bottom of the account information page under "Current Activity.")
We cannot, under any circumstances, print an invoice. Invoices are only used when the amount of goods/services purchased is known prior to payment, which is not the case with a prepaid service like ours. We do offer detailed records of past account activity via our UI which typically provide the same information as would be contained on an invoice.