Thanks for the quick reply.
So it works fine with Twenty19, but when I revert back to Fundrize and disable all plugins except Woocommerce and this plugin, the problem remains.
So it looks like a conflict with Fundrize unfortunately.
I get a sense that it is something very simple, as it almost works! I know you can’t take responsibility for every integration, but is there anywhere in particular you can suggest I can play around to try and get this to work?