We're working on the ability to automatically send package purchase receipts to your client through DesignFiles.
In the meantime, you can send a receipt from Stripe (the payment processor). Here's the steps to do that:
Login to Stripe (with your previously set password) > http://stripe.com/login
Select "Payments" on the left
Select the specific payment you wish to send a receipt for
Scroll down and select the [ Send Receipt ] button
Enter your clients email address (if needed) and click the [ Send ] button
They'll receive an invoice by email (as seen below)