I'm leaving the question so that perhaps someone else can find the answer more quickly.
I'm new here, so please excuse the probably basic question:
My payment service is PayPal Advanced, and I note in the discussion about it that there may be some problem with the cookies needed for the pop-over and PP Advanced, but I can't even get to the point of testing that.
My (still under-development) site is http://firstname.lastname@example.org
Thanks for any help!