We have multiple buttons (for different services). We want to add an email address to a single button to send sale receipts to. (now it is only sent to the email address of the PayPal account) Is this possible with advanced variables (or other way)?
These types of buttons are a deprecated legacy integration method, so you should be transitioning away from them regardless. But to answer your question:
First add and confirm new email(s) in the PayPal account, https://www.paypal.com/businessprofile/settings/email
Then create a new button and when creating it in the bottom of Step 1 change the email address.
If you were to create the button yourself (not hosted with the PayPal button generator) you would set the email address in the business parameter that identifies the account to receive the payment.