With Swreg, if your purchase was made through the Swreg payment platform, you can get your invoice/receipt from the following link:
With Avangate, Avangate myAccount is created automatically when you first place an order through the Avangate system, you could login your Avangate account at https://secure.avangate.com/myaccount/to check for. When accessing Avangate MyAccount for the first time, you need to set up a login password.
With REGNOW, go to the page https://www.regnow.com/order_lookup.html , enter your order number and email address, you should find your order with a link to your receipt. If not, please contact the REGNOW support at firstname.lastname@example.org.