I had a mail order business (side business) for 13 years selling collectible card games like Magic the Gathering, Pokemon, etc. I used a scale and I printed labels out on my printer. I met 100s of customers at show that thought that I was running a full time business because of the volume that I sold. I emailed the customer after receiving payment (same day) with the tracking number, the same day and explained that the tracking number from the post office doesn't activate for 24 hours. I mailed thousands of packages and yes occasionally something didn't get shipped out. When a customer followed up with me I'd apologize, ship the package the fastest method possible (at my expense) and refund part of their purchase price or throw in $5-10 in free promotional cards.
We're all human so we're bound to make mistakes. The best way for the seller to handle it is to apologize and offer the buyer two options; full refund or ship next day Express Mail at no additional charge to the buyer. If the seller hasn't responded within 24 hours to your call you tell us who he is so we can avoid doing business with him.