Too few pixels, the wrong shape, no bleed, a QR code that doesn't scan. PrintSanity fixes most of it in a minute, before anyone at your shop opens an editor.
It won't vectorize a logo or un-flatten layers. That's still a person's job.
At the counter: drop in the file the customer sent, pick the size, download the PDF.
Before it reaches you: put printsanity.com on your file-upload page or in your reply to customers ("run your file through this first"), and files arrive fixed.
Use it as much as you like. In return, tell us what it gets wrong and what it should do next: buyselllab+printsanity@gmail.com