You can easily find and review all your past transactions with MarineTraffic. Just enter the My Account section, choose the Billing option and click on the Purchase History pane.
If you had made any purchases in the past, all of them would be shown here. Transactions are ordered by date starting from the most recent one. Each transaction has its own unique Order Number while you will be able to find the price paid for it and the date of purchase.
Clicking on the respective button at the right end of each transaction's record and you will also be able to get a printable invoice for it. The invoice pops-up on your screen and you can get it printed by just clicking on the Print button located at the bottom of it.
Note: Make sure you have provided MarineTraffic with correct and accurate information during your registration process so that the invoices will also be accurate.