Well good thing the vendor offers that option then! I'm sure he's done the math on orders delayed by UPS to make sure the risk is worth it given his live arrival guarantee.
Most of my fish are from Dr. Reef and have been nothing but healthy and eating well. While I've had a couple losses here and there due to shipping and my own inexperience at the time that's nothing I can fault the vendor for. They've been nothing but exceptional in staying in contact with me even on delivery days when I've had questions or concerns during acclimation, etc. and have always done right by me.
In terms of getting shipped additional fish that may be a little more on the sensitive side, I don't have much of an issue with although I know my case may be more niche than most. It's somewhat of a process getting shipped fish here in Hawaii. For one, our LFS is limited to pretty much two (one being recently opened this year) stores not named PetCo, and I would not touch those PetCo tanks with a 10 foot pole after seeing firsthand how bad the condition of the fish and tanks are. One out of the two LFS is notorious for having ich and uronema in their system, although it may have gotten better over time it is still hard to completely trust. That pretty much leaves you with a very limited options for both stores and species options.
Now if you're getting fish shipped to you via an online vendor, you have to first clear it with the Department of Agriculture via a permit and a processing fee where they'll have an agent review the species that you are planning to bring in, etc which can take a month or more to get back. While this is a minor inconvenience at most it can understandably add up especially given the nature of this hobby with things happening during transit and DoAs. To get a replacement on a DoA would require another permit and then even more waiting for it to clear, so you can get how it can add up.
All in all I'm in a fortunate position to where I can (and have, in the case of a banggai cardinal) give the extra fish to a friend who also has a tank or go through the Facebook/LFS route with the one LFS I can trust on island so I have no issues with getting sent and extra fish as a sort of insurance. I still do have some store credit with them that I am planning to use once my tank settles from some issues I've been having.
2 days ago we added the question of accepting extra fish. Since then we received 163 orders and their answers (till writing of the post) and only 3 out of 163 dont want extra fish.
Not to add to the drama, but I'll chime in and say that out of everywhere I've ordered from (including some other high end quarantine sites), I get consistently fatter and "better" fish from Dr. Reef. Not to say anything bad about the others, but I can't say that I get fish from anywhere else that so consistently enter the display, swim around, and eat. I've had bad luck with Copperbands for various reasons with tank changes, aggression, etc, but every one I've gotten from Dr. Reef has eaten frozen. I'm not saying he's the only way to go-- there are other great quarantine sellers as well, but my experience has been nothing short of an A+ and I've spent well over a thousand dollars.... probably more.... with them.