I'm not in business, and use USPS with insurance for small packages for my infrequent shipping, and for substantially less then we are charged by our hobby suppliers. What I think they do is to figure a worst case shipping cost, then charge all at that rate regardless.
Meanwhile, down at Amazon, Home Depot Online, and others, free shipping is the norm for items that value $25 or more including tracking, and the carrier is responsible for safe delivery. I doubt many of us buy less than $25 worth of car parts in one package, and our suppliers do the opposite anyway, increasing the shipping charge along with the value of the purchase.
For me, shopping for anything and making a decision involves all cost, including shipping and state tax. Our hobby suppliers have us over the proverbial barrel in that we are happy to have found an item from anyone and will gulp a lot, but we'll pay.