You should accept the $15 frag and end the negative feedback.
You are forgetting that this is as much your fault as the sellers.
I am completely serious too, before the rebuttals begin I will explain.
1. You chose to buy Online.
No matter what every purchase of livestock Online is a risk. Too many things can go wrong! Shipments get delayed, shipments aren't packed right(heatpacks?), deliveries come unannounced, substitutions take place(swap corals for what you ordered), corals come dead(arguments about how it left alive ensue), and on and on and on!
2. If the coral is not a WYSIWYG, then you gambled again. Anything that is not sold as WYSIWYG is subjective to the sellers idea of what they are selling as fair. Just because you got something smaller than you expected does not mean the seller was being deceitful.
3. The seller tried to make amends. He sent the flame scallop for 2 reasons. 1, it was going to die in his tank anyway and 2, he was doing PR for the late delivery.
4. After you disputed your sale/purchase(which you have every right to do), he offered another peace offering. He is trying to comply with you.
Are sellers scamming with their ads? Sure they are.
Do you as a buyer have to assume that you are taking a chance when purchasing such items? YES you do.
To rectify, use popular sources for your coral purchases and only buy WYSIWYG items or have the seller email you a pic before you purchase.
If you are trying to win bids for the cheapest price, then you get what you pay for.
My point is you have to be aware of sellers and expect less than optimal service from sellers who are not commercial entities.
I hate to not be on your side, but you have a shared responsibility in this debacle.