How can I add a Paypal, visa, mastercard payment logo to my invoices?
Depends on how and where. You could use the invoice customisation and add some logos anywhere on the page pretty much or you could just add them to your logo or you could add them as a custom footer.
In general, you want to look for invoice customisation and then you should find loads of forum posts and knowledge base articles on how to achieve this.
It’s basically a case of adding custom html tags in the footer. There’s a similar discussion on the following two posts:
http://community.quickfile.co.uk/t/adding-trade-association-logos-to-my-invoice/1835
http://community.quickfile.co.uk/t/trade-association-logos-in-footer-area-changing-size/2127