I am currently testing the payment for subscriptions on my site.
In Sandbox mode after I make a payment PayPal tells me the payment is confirmed and I receive an email from my site that I've received a new payment. But then there is a button on the confirmations that says “Click Here to Complete the payment”. If I click it, I get a second email telling me that payment has been received. It is only then that I get taken back to the payment success page on my site.
I am unsure if this is PayPal or WPUF issue. I'm also unsure if it is an issue just in the sandbox, or if customers will encounter it when we go live.