Hi girls! I have been selling signs on Etsy for a few years, and the easiest/cheapest way I've found is via USPS. I ship from home, and print my labels from home as well. If you order priority boxes from USPS.com, they are all FREE. The best and cheapest way I've found to ship is Priority, because you can use Delivery confirmation and also the price is not much different (if at all) from parcel post. Trust me, I've researched this a LOT. I ship my smaller signs under 13 oz. First class mail. I now use stamps.com to ship from home, but the shipping through Etsy works just as well.
As far as shipping supplies besides boxes, I order packing paper and labels from Ebay. I'm glad this group is getting more active, I'm happy to be able to talk with others who sell signs to get ideas! :)
Jackie