I need a receipt / invoice for my purchase.

No problem! You can download a receipt from within your Account Settings. 

1. Login to your LitCharts account.
2. Click on your username in the top-right corner of the screen, then select "Account Settings" from the dropdown menu.
3. Click the third tab on the Your Account page, labeled "Invoices." 
4. You will see a section titled All Invoices, and from here you can download any invoice you need by clicking the "Download" link on the right.