If you are using a Remote Checkout method like PayPal Payments Standard, there will not be an option to send the reciwt using Universal Email, that Option is only available using a Local Checkout method like PayPal Payments pro.
Payments p[ro is 2 services:
1) Direct Payments (The User enters credit card details on your site)
2) Express Chekout (The user is directed to paypals site to select payment method, and is then returned to your site to confirm the transaction)
If you want to allow only checkout through paypal, and still send a Receipt using UE, Select to use paypal payments pro as the checkout method, and remove the credit card form on the checkout page that is created and only use the Checkout with Express checkout button.