I used to be deep into leopard gecko breeding. The weight at which it is deemed safe to ship is arbitrary and most often about a seller's comfort level. 10g, 15g, and 20g are commonly cited collective "standards" in the hobbindustry.
I would find it pretty uncommon for an experienced seller to be comfortable with shipping a 5g gecko unless maybe we are talking about two exceptionally experienced keepers shipping to one another because of multiple factors being coordinated that tipped the scales towards doing it, but even then it would not be anywhere close to typical.
In terms of things like temperatures, it would be no more safe to ship a 10/15/20g gecko than a 5g gecko, but the one factor that is going to be worse at smaller size is concussive force. When an entire vital organ is built at the thickness of a hair or a single ply of tissue paper, the force of a hard impact has potentially greater consequences. While it is an extreme example, I was once t-boned while driving and taking a very young tortoise with me from point A to point B rather than leaving it alone while in a fragile state for a week or so (so much for trying). It was to my right side and cushioned in a well-packed container. The force of the impact transmitted through my body from left to right and the tortoise was in the path of horizontal transmission even after the wave would lose energy through my body. I was fine, although slightly pissed off. It died while waiting for an officer to arrive at the scene. I opened it up and its internals, paper thin, had been shredded. It is an extreme example, but illustrative.
Would I ship animals that small? Under a collection of multifactorial specific circumstances, I might. As a standard, I probably would elect to wait until the animal was larger for this kind of reason and also to be sure the feeding and passage of waste were where I wanted them to be, but we all have our own thresholds.
I understand hesitation to ship back. I do. Still, it is an expected part of a refund process. There can be a good faith gesture here of waiting until the animal achieves a higher weight, but that means time, work, and risk for the buyer that it might fail in the process. From a business perspective, I would not recommend waiting. The bad event is the seller's responsibility, but practical solutions that may not be feel-good solutions have been proposed (or stipulated).
As for the giant genetics, they are not a guarantee even when legitimately involved. Contrary to all of the original junk information put out there regarding giants and "super" giants, it seems to be more about lineage and individuals within a lineage than a single allele (as was originally the assertion). One can have "super" giants produce geckos that grow up to be normal size. Or on the smaller side of adulthood. Big geckos tend to make more big geckos, but sometimes not. And what is interesting is you can have a normal or smallish animal of a giant lineage then produce giant (or so-called "super" giant) offspring and grandoffspring down the line. Sometimes a giant lineage animal plainly does not blow up. Especially during the first year. If that happens, its frame seems to be set at a normal size even though it can still pass on desirable genetics to the next generation. I had an amazing male leopard gecko that I produced like this from "super" giant parents. The "super" term is a bastardization of the genetic idea at this point since it is more about size and weight class these days than the function of the super form of a single allele that appears not to really be at play here anyway.
The seller is normal to expect a return prior to refund. The buyer is normal to care about the welfare of the animal at this small size. It would have to be a return-for-refund or some kind of compromise between the two parties at this point. While the seller could just say hey, free gecko because of his "bad" and refund, that is not expected practice. Some would do it. Some would not. He would not be obligated to make that choice. They could wait until it is a little bigger to send it back like I mentioned earlier, but that comes with risks as I also mentioned earlier. They have to figure out what they are willing to accept and then try to move forward one way or another.
The seller should get his operation under control, though. This should not have happened.
