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 will just keep going along just fine.
If the user might not end up back on your site for some reason, run a cronjob that tries to verify transactions created in the past day/hour/whatever.
An issue like this doesn't have to, and really shouldn't, cripple your business.
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 will just keep going along just fine.
If the user might not end up back on your site for some reason, run a cronjob that tries to verify transactions created in the past day/hour/whatever.
An issue like this doesn't have to, and really shouldn't, cripple your business.