Sorry to hear you went through that. It sounds like a pretty horrible combination of seller incompetence, USPS delays, and bad luck with the weather.
I always told people that plants would die if they were exposed to freezing temperatures, and that they should act accordingly, and consider not placing a bid. I was extremely blunt. I was likewise blunt about cloudforest plants having zero chance of survival outside in the South; that routinely got ignored, and as I say was the bigger problem--lots of money was wasted. At some point it makes sense to allow people to act as they choose. Possibly one or two asked about heat packs; it certainly wasn't a significant number. As I say, I never offered one, and if someone insisted I'm sure we could have figured something out. But it never came up.
I don't think one can expect a seller to know the weather forecast at a destination (and that can be unpredictable) or at points in between (where?). That's the buyer's responsibility to communicate, if there's a problem. It's certainly inconceivable that a seller would suddenly include free heat packs in all orders once temperatures dropped below freezing at some point in the country in the Fall. I always sold individual plants in auctions, and it was extremely rare that I would send more than one plant at a time. These were typically auctions in the $10-$20 range, sometimes a little higher. Free heat packs would have destroyed even a small profit. Among the outliers pricewise were Philodendron 'Pink Princess' plants, which back then went for close to $100 for small ones. I was afraid I'd hear that one got frozen or otherwise damaged, and I'd have to refund that money. It never happened.
There are many factors that the seller has no control over. If they very bluntly state not to buy the plant if it might freeze, then that's what the buyer should do. I haven't sold much in several years; a few plants locally in the last couple weeks. The issue came up twice this winter. Someone in Lubbock wanted to buy a plant. I checked their forecast and it was for freezing weather. I asked if they wanted to postpone until it warmed up. That was the last I heard from that person. The other was someone in Virginia who I already knew. She asked me to save one, to send in Spring.
I did buy a plant online today, a Nepenthes. The seller says: "we only guarantee live arrival with the purchase of a heat pack and express shipping if temperatures are below 45F in your area." (I'm not going to do that). That's one way reasonable way to handle it. When I had Ebay auctions, I did not void any return guarantee if the plant froze. I just warned the person not to bid on it in the first place.
If communication breaks down between buyer and seller, that's obviously bad. Post office delays will happen, and of course may make heating packs pretty ineffective. I don't order much by mail; we have a lot of local sources. I did have a decent sized Begonia order a few years ago take 10 days to get here "Priority Mail" from Texas. Surprisingly, everything survived, all in good shape. I wouldn't order those in winter, though.