I have got it working, using the plugin Simple Cart & Buy Now version 1.9.3 by Ben Luey.
The plugin WP Simple Paypal Shopping cart is not usable as the shortcodes are not registered by add_shortcode()
so i have no control over when the shortcodes are rendered.
You will need wppa+ version 4.9.11.
You can see a simple preview here. Also, after adding to your cart, check the checkout page.
It will require some css work.
There is one issue: the return url (after adding to the cart) is to the page, ignoring where we are now ( i.e. ignoring the querystring ).
I will see if i can make an interface for wppa+ specificly, to cope with this. (in the future, i.e. not yet tomorrow).
Please ignore the warning messages on the demo pages, this is debugging only.