If you have valued livestock, you should send overnight IMO. If the buyers are not willing to pay for it, just kindly tell them that's how you ship and you do it that way to eliminate die-off and uncomfortable situations if/when something arrives DOA. I, for one, would never ask a seller to send anything 2 day. If they (buyers) are trying to save a few bucks, then this hobby may not be for them...lol
I have had stuff shipped USPS next day, UPS Overnight, and Fedex Priority Overnight. I, personally, ship everything Fedex Priority and that is all I offer to a potential buyer. If they buyer is not interested in Fedex Priority, then they are not interested in the frags...I spend too much time growing and feeding my corals to take a chance on 2 day shipping. Not to say that it won't work...just my two cents.
I always guarantee an arrive alive and always want to make sure the buyer is happy with what's in the bag before I consider the deal closed.
Bottom line, you ship your corals how YOU see fit. You want to eliminate as many external variables as possible - and a 2nd day in the bag is definitely one of those variables.