Unlikely they would pay for a replacement.
A dent in the bumper doesn't affect how the truck works.
If you bought a truck with scratches in the paint, you can't go back the next day saying to fix it beucase you didn't notice it.
Certified pre-owned means nothing except it has an additional warranty vs a regular used car\truck.
A dented bumper wouldn't be fixed under a regular warranty, so I wouldn't expect them to do anything about it assuming then even knew.
They will just say they never removed the license place cover to check for damage and they didn't know.
You already bought it so it's unlikely they would do anything with the price, but maybe you could get them to "knock $500 or $1000" off and give you some money back.
I did that before when I was buying a used SUV, saw some problems with the rims and we knocked a few hundred off the sale price (before I paid).
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$570 + 30 bucks shipping.
Just send them your paint code PXJ when you order.
I got a painted bumper off ebay before with no problems in the past.
If you prefer Amazon here is the link for a painted to match bumper there $760 there vs $600 at Ebay.