eBay had (maybe still has I don't use it) that problem too. Sellers would list something with a Buy-It-Now price of $5, but when you got to the last step, you'd see that shipping was $25.

At the time we suggested that they limit shipping fees to no more than 50% of the total cost of the item, but it never got implemented.

I just checked and it looks like their compromise was to set max shipping rates for different types of items: https://www.ebay.com/help/selling/shipping-items/maximum-shi...