I just filled in the PayPal standard settings, including my PayPal email address, and I set the gateway to PayPal Standard. But, when I visit http://plumberjax.biz/wpi_invoice/inv-2428/ to see printed invoice and then press the green “pay for invoice” button, I still get a blank page.
PS: I have the miscellaneous settings set to allow not being logged in to view invoice. I tried changing that setting, but it didn’t solve the problem, so I changed it back.