Regarding of how many payment gateways you have activated, only the Paypal logo shows next to the "Payment Options" line in the invoices footer.
This is quite confusing because you would expect that by clicking a Paypal button a Paypal page will pop up. Instead, this links to a Zoho portal page, and, in there, you can choose to pay that invoice.
Maybe using a more generic online payment icon would fix this.