s2Member Pro, PayPal button with custom success page does not send account creation email

Attempted to change that boring “Thank you! (you MUST check your email before proceeding).”… page that is the default success page when returning from a subscription paid by PayPal (and probably others).

I created a new page in WordPress, and added success=“that page” to one of the buttons on my test page.

After successful payment, I got redirected to that page, but I did not get any signup confirmation email with the account creation link. This works when the success parameter is removed.

So, what am I doing wrong ? How do I correct it so I get returns to my own success page (as I do when I have the success parameter there), and receive the signup confirmation email (as I do when I remove the success parameter) ?

In case I want to create a dynamic success page, telling the new subscriber which subscription was selected and maybe more (which I now will do by using three separate pages), how do I pass and use the variables in the success page ?
The description under “PayPal Pro Forms” -> “Custom Return URLs Upon Success” gives only one example on how to send one variable to the success page, but does not tell how to use it.