Receipts can be customised by copying the receipt.php template in your theme directory.
The POS receipt print templates reside in the includes/views/print/tmpl-receipt.php file of both the free and the Pro plugins. Receipt templates can be customised by creating a woocommerce-pos/print/tmpl-receipt.php file in your theme directory. The code for both templates is included below.

Basic Receipt Template

Pro Receipt Template

The Pro Receipt Template allows additional information from the Stores admin such as logo, store address, opening hours and special messages.

Customising the receipt date

WooCommerce POS uses moment.js to localise the date strings. The default date format is "MMMM Do YYYY, h:mm:ss a", eg: May 31st 2015, 7:20:44 pm. Please consult the moment.js documentation for more information on customising the date format.

Order Properties

WooCommerce POS uses the json output from the WC REST API to populate the order receipt template. The WC REST API docs show an example of the standard json output. Some additional properties have been added by WooCommerce POS using the woocommerce_api_order_response filter.
The tax portion of the cart discount
User ID of the Cashier
First name of the Cashier
Last name of the Cashier
Payment gateway success or failure
Any messages from the payment gateway
Redirect URL for offsite payments, eg: PayPal Standard
Any gateway specific messages, eg: Amount Tendered and Change
Add tax amount for each shipping line
The tax portion of the subtotal