A fairly accurate scale is very helpful to have. One can get one that weighs up to five pounds from USPS for around $25.00.
One can give an accurate weight and cost to the prospective buyer.
(One can also use it to weigh food portions if one is trying to keep an accurate food diary)
It's also worth while to be able to print the postage through PayPal or some other service
http://www.quiltingboard.com/t-101275-1.htm
I was very resistant to doing it this way, but now that I am, it saves the buyers money and is more convenient for the shipper.
For USA addresses to USA addresses:
I use manila envelopes when the fabric plus envelope plus plastic bag weigh 13 ounces or less. Then I mail it using first class postage. I also enclose a file card in the inner package with "to and from" on it. "From" includes my board name, real name, and home address.
Flat rate envelopes (FRE) work well from over 13 ounces to as full as one can get them. They come in three versions (that I know of) now - the regular size cardboardy one, the legal size cardboardy one, and the padded one. The post offices only carry the regular size cardboardy one - the other two need to be ordered on-line through USPS.
There is a lot of Priority Mail mail packaging available. Sometimes the Flat Rate is the best deal - but not always.
The packaging is "free" - the postage is paid at the time of shipping.
One does need to pay attention to what the labeling is on the Priority Mail packaging. If it does not say "Flat Rate" somewhere on the package - one will be charged the "regular rate"
I've found that there is a "gap" between the FRE capacity and the medium size boxes. I use the Tyvek priority mail envelope for those. For those the postage cost varies from $5.00 to $10.45 depending on the weight of the package and where it needs to go.