When logged into the portal, click "Account" on the left hand navigation.
When you are on your account page click on the "Balance" tab at the top.
From you balance page you can download your invoices by first changing your filter setting to Credit/Debit Card Transaction.
From here simply find the transaction for which you want to download the invoice and click on the 'Download PDF Invoice'.
Please note that only invoices from credit purchases can be found here. If you require a copy of your monthly VMN invoice, please email accounts@voodooSMS.com. You can also try searching this email address in your inbox, as VMN invoices are automatically emailed to you monthly.