USPS flat rate boxes cost the same from same city to across the country,
Yes, this.
What I have always done if not using Priority Mail flat rate boxes (which is rare for me) is box the item up, not necessarily sealed yet, and calculate the cost from here in Seattle to a zip code on the east coast, maybe in Florida. Once I know that cost, I know if I need to adjust the asking price to cover that cost or not if it is an acceptable cost/loss to me.
If I want to buy something online I don't mind covering the shipping costs if they are reasonable.
Shipping is never really free IMO. Just say shipping included. I doubt it would put anyone off if you were to ask $145 for it shipping included to cover the cost.
Using a flat rate box, which is a free box from USPS gives you a set cost dependant on the box size, for shipping anywhere in US and it's possessions essentially regardless of the weight. It is hard to beat Priority Mail!