I've had to do two large coupons. Both times I let ap know about it and why I had to do it. I figure it was better coming from me with an explanation than showing up on a report later.
I think the most important thing, is to not accept any printable coupons over $10. And remember, we don't accept coupons from the internet for free items either. I've seen guests bring in coupons for free xboxes and video games and such. Those are typically fraudulent.
Real High Value Coupons, or Free Item coupons, should be printed on some sort of special paper, and contain a hologram. However, if it's a Target.com coupon, these should be simply verified (that the guest purchased the correct item) if it's high value.