A receipt is not required to pick up your tickets at will call, it may however be helpful as confirmation the order was processed successfully. You are only required to bring the original credit card used to purchase the tickets and your photo ID.
To print a copy of your receipt anytime, from any computer, simply log into your account and click on the "Purchase History" link on the left hand side of the page. Click on the transaction ID and press the "Print Your Receipt" button to open a .PDF of the receipt. If you do not see the order in your purchase history it has not gone through, please try purchasing the ticket again.
If you purchased print@home tickets you do need to print out your tickets to present, otherwise you will not be able to enter the event.