Tell your buyers (in your listings, invoice and payment confirmations) the estimated shipping time for the shipping methods you offer. But don't use the U.S. postal service or eBay estimates - especially around Christmas. If the post office says that media rate or parcel post shipping time averages 5 days then indicate 10 days, or 10-14 days.
Package your items carefully, in many cases sealing them in protective wrap before placing them in a padded mailers or boxes, with enough breathing room so that they can survive rough treatment. Remember that postal employees often throw packages around, drop heavy packages on top of smaller ones, and leave packages out in the rain or snow.
Don't attempt to save 50 cents on packaging materials by insecurely packaging your items, and letting your customers decide whether or not to buy insurance. Usually they won't, but if items are damaged, customers will blame you, want a refund from you, and may give you negative feedback, whether or not they insured their items.
You can save money on postage costs for priority mail by using the free materials the U.S.P.S. provides. You can save packaging materials that you receive for your own orders, or pick up materials for free from local stores who recycle theirs.
Personal recommendations: I recommend joining your local Freecycle.org list and posting a notice for packing peanuts. Many people have bags of styrofoam peanuts that they no longer need. Also, an excellent low-cost online store for purchasing shipping supplies in bulk is Uline.
