My Petco is the same, they always have good morphs. We got the Lucy for 250 at the reptile rally sale. That sounds a lot like my boys. It's firm and immobile which is concerning. Thinks like hernias are able to be pushed in, so I'm not too sure what it could be then. He eats just fine too and it hasn't gotten any bigger.
We may end up taking him into the vet, but it sounds hereditary with what you said. I had plans to breed him when he's older but now I'm worried about producing offspring with that lump.