You can view your transaction history and print receipts / invoices for all past transactions on the Billing info page of your account on the TripIt website (not available on mobile).
To access this area, first sign into the TripIt website, then, in the upper-right corner, click the arrow next to your user name and select Settings. On the left side of the page, select Billing Info. Then select the Billing history link to view your transactions and click Print Receipt for a printer-friendly view of the transaction.
Updated: Feb 12, 2019