OK, that pdf is very helpful. It seems I've misunderstood the PayPal options. I had thought that PayPal Express was the version you could do with the free (no monthly charge) version of PayPal. Does PowerStore work with that version? What's it called, and is there documentation on it?
At the moment my admin panel seems to be intermittently malfunctioning and won't let me go to the payment gateways page to see what the options are, or I could be more precise. Not sure what that's about.