Shipping domestically is a challenge for new eBayers. Personally, I have made all the mistakes. Now that eBay charges the Seller a fee based on shipping charges, the seller should be saavy about how to ship for the least cost. The customer also benefits from this knowledge when the seller gives them the best possible price on shipping.
Get a Postal Scale - it is worth the price. I purchased mine right from the official US Post Office website Yes, it costs around $50, but weighing and shipping your packages with labels through eBay saves you money. I have always shipped that way.
When I get packages from people actually going to the Post Office, I have realized that eBay is offering a discount on the COST of the shipping.
An example is a Small Priority Mail Box - on eBay I currently pay $5.00 which includes delivery confirmation. I print the label on my printer and tape it to the box. A friend takes the box to the Post Office. No hassle waiting in line, just drop it off - done deal. I recently received a Small Priority Mail Boxes that had been processed at a post office, the postage paid was $5.90. Wow, use eBay - save a buck!
USPS First Class (United States Post Office) is best for items less than a pound. Weigh the item and enter the ounces into the Calculate Shipping Weight on your Auction Item - each location will get the appropriate cost but essentially it is the same for all domestic locations; a 3 oz package is currently $1.75, a 6 oz is $2.26, an 8 oz is $2.77 each with Delivery Confirmation. The package must be over 3/4 of an inch thick. You can buy padded envelopes - but I just use a regular 5x7 white envelope with styrofoam inside (part of an egg carton works, anything over 3/4 inches thick that protects your item.) Learned this tip from a fella selling ties here on eBay.
Do not expect special handling for your package - EVER !! Tape and secure your package so the item will not fall out and the package cannot be opened by snagging on equipment as it goes down a conveyor or is tossed into a container. The items in your package should not move around - include enough packing so it does not rattle when you shake it.
Once you have created the package, including tape and packing, weigh it. Then add an ounce for the final label. That is the weight to post on your Auction Item for calculating and what you will print on the label once it sells. You can also add a handling charge to the item so you cover eBay fees (usually under $0.20).
If your item weighs more than 8 ounces, consider a Small Priority Mail Box for a flat rate of $5.00 or a Priority Mail Padded Envelope for the flat rate of $4.95 currently. Prices may change. These are the costs as I write this. Be sure to pack the items well (no rattling). Tape the box and envelope so the flaps and all edges are covered. This is not shipping by weight - make it secure.
If your item is breakable - use bubble wrap and be sure it does not rattle - impact inside the box will be similar to the driver's head hitting the dashboard in a headon crash. (I learned that here in a Guide on Shipping Glass - it was very informative and I highly recommend reading it.) Put packing inside the item as well as around the outside, if it is appropriate. You don't want open spaces inside the box. Everything should be cushioned.
Good Luck with your Business and Have a Great Life!