It looks like there's a stop-gap between check out and the redirect to a "thank you" page: PayPal.
When you complete your purchase of a product, you're presented with PayPal's standard completion page. On that page there is a link that says, "Return to (Company Name)".
It's not until you click the link that you're re-directed back to whatever page you designate in the Simple PayPal Cart plugin settings.
Is there a way to automatically redirect a customer after purchase instead of them having to click that link? We can't even add any kind of instructions to that PayPal page that says, "Please make sure to click this link to return to our page to book your event!"...