Once you have packages over 70 lbs (although some carriers will go a bit higher), or above a certain size, carriers such as UPS, FedEx and the U.S. Postal Service can no longer carry them. Their vehicles are not outfitted for the weight of heavier packages which need to be handled differently. Each carrier has a weight and size threshold that is their limit. You can check their websites for exact specifications.
If your package is above the weight and size threshold, you need to find a carrier that will ship it for you. This situation generally results in shipping by common carrier. Common carrier is a network of full-sized trucks that ship all over the U.S. This method of shipping can be very costly, because it requires special handling. Common carrier trucks often hire union workers, have specific job requirements and need to use more equipment to move the heavy packages. Diesel fuel is expensive which is what these trucks run on. All of these factors add into the cost of shipping your item. Some shippers are national, but most are regional. Some specialize in specific types of shipments such as food shipments which need refrigerated trucks.
As you look into common carrier service, do your research carefully or you will find the cost to be prohibitive.
Items that are large or heavy enough to ship common carrier may require special packaging. Often these items are shipped on a wood pallet and must be attached to a pallet before the shipper will take them. Because these items are handled more by machines and are heavy enough to break most cardboard, they frequently require wood crating. Discuss the best packaging methods for common carrier with your shipper and plant manager.