The plugin and purchases were working fine at http://www.energyquarter.com/passive-house-tutorials/
But now no purchase can be completed and the potential buyer returns from PayPal to discover, instead of the promised material, the message:
"There was a problem finalizing your purchase with PayPal. Please try again."
Trying again does not produce a different result.
Is there a bug? What can we do? We've double checked the API details, everything appears to be in order, except that the final step does not succeed.