I print my own shipping labels and save at least 50% over what most sellers charge. I just shipped an 88lb amp for $65. I self insure. If you package well, you will never have a claim.
I always price things shipped. That makes it much easier for buyers and things sell much faster.
As a buyer, my offer is always "$XXXX shipped". I want the seller to understand that he is responsible to deliver the item to me. I make no shipping decisions so that there can be no "but I shipped it the way you wanted me to." The only thing I tell the seller about shipping is, "no hurry, take whatever time you need to ship it safely."
To estimate shipping, I sometimes just put in my college address in Chicago and calculate shipping from CA to Chicago to give me an idea.
I ship using paypal shipping which is now ship station. there are other ways to buy postage online including directly from USP, Fedex, or USPS.
Jerry