Last time I shipped something big and heavy, I was able to look up cost on line at UPS and it was either 100% accurate or within $ 2.00 or so. All I recall is they were big wheels and tires and I was a go-between helping out a 6MT member in CA that wanted tires and wheels sold in Denver and listed on ebay.
UPS ground, 5 days or so on other things like boxes wasn't too bad or at least the cheapest. If you ship books as 'media' you can really get cheap but they will inspect it to make sure it's 100% books.
On brakes or cars parts, you can really only value them by what new ones are with (warranty) and shipping and negotiate from there. My interest would perk up on any bigger ticket item that is $100 or more if I could save 20 or 25% from a reg parts store.
'04 G35 6MT, DG Coupe, Prem/Nav, purchased 06/2011 @97k mi / 119,650 as of 07.06.16