The invoice for the prior period is available on the Finance page -> Invoices in your SSP account by the 10th day of the current month (e.g. the invoice for July will be available on 9-10th August).
Please, keep in mind that for each deposit you add to your account there is a Prepayment Invoice which is not a Tax document. The prepayment Invoices can be found on the Finance page (1) -> Billing History folder (2) -> option "Payments" (3). By clicking PDF, you can download the prepayment invoice.
The payments you have made can be filtered by date. You can either use the presets (4) which will show the payments for the following periods (for today, 7 days, 30 days, the current or the previous month) or put specific dates in the date field and click Set (5). By default, the payments from the first to the current day of the current month are shown on the page. 
By clicking Add Funds (6), you can deposit money. Please note that the minimum investment is 100$.