After entering a transaction in a Contract, hover over the transaction and select the "Pencil" icon on the far right of the row to edit the transaction.
The "Edit Transaction" window will pop up. At the very bottom you will see 2 options: "Email Receipt" and "Print receipt.
Select "Email Receipt" to automatically send a receipt to the customer via the email address you have on file. See this article on how to edit contact details to add an email to a contact.
Select "Print Receipt" and you will be taken to a page where you will be able to print a physical copy for the receipt of this transaction.