If one plans to sell several things, it's worth the expense of buying a fairly accurate scale. I purchased a diet scale that weighs up to 4 pounds that works well for me -I think it cost around $30 - I think it's supposed to be accurate to 0.2 ounce.
(I originally bought it to weigh food portions more accurately than I could approximate a serving when I was keeping a food diary - which does work when I do it!)
Sometimes the Priority Mail Flat Rate packaging is the least expensive way to mail something. Sometimes not.
Remember to factor in the cost of packaging if not using the Flat Rate options and the cost of taking the items to the post office.
I still hate to pay for shipping - but when I remember how much it would cost to drive all over the country looking for something - along with the time to get there and back - it eases the pain somewhat.
I think it is reasonable to look at "replacement cost" for a similar quality fabric when setting the asking price.