I've been using USPS Click-n-ship since its debut a few years ago because I sell and buy on eBay and it makes it easy to complete a sale and then drop off the package. I have a postal scale that lets me accurately weigh my parcels and this has helped enormously in streamlining my shipping. Over the years I've learned a great deal from my local post office staff as to the proper way to pack and ship, especially for the international markets (filling out the right customs/declarations forms are critical). In addition, I like the ability to track my packages on the www.usps.com website. Delivery confirmation is free when using Click 'n ship, also. The website is chockful of information on shipping--including zipcodes.
UPS is good for large packages, however, their facility isn't near me and daily pick-ups are costly. I have dropped UPS return shipments at my local Staples on occasion (if the vendor includes a return post paid shipping label for the carrier).
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