Where can I download my invoice?

All your invoices are available on this platform: https://wallabagit.invoicing.co/client/login. If it's the first time you access to it, you have to use the email used to pay on PayPal, PayPlug or Stripe. Then click on "Forgot your password?" to set a new one.
Now, you have access to all your invoices. 

This platform is an instance of Invoice Ninja, an open-source invoicing software.