You can make an API call to check the status of a transaction very easily. When my users are redirected back to my site (thanks page, or similar), I check if their transaction is completed, if not, I kick off an every-five-seconds check while asking the user to hold on while we talk with paypal. I will eventually fail after some number of checks of course, but this means PayPal can stop sending IPNs and everything wi…
My favorite Paypal annoyance is that for split payments there's no way to verify that a seller's Paypal account qualifies for split payments (has to be a business account and verified), without trying to send a payment. At least, that's the way it was last it mattered to me.