Harry_Morant gave some very good advice and it would be hard to expound on it! However, if you live in the U.S., you can get some shipping boxes and envelopes from the Post Office directly even if you are just a part time Seller on eBay. Now, if you hold a Business License (I think it is required), you can go to the USPS Website, signup, login and order shipping boxes and envelopes to be delivered to your' door and they are free. I only ship via the USPS and keep an assortment of "Flat Rate" and "non-Flat Rate" boxes on hand to use depending upon where the item or items is/are being shipped, I compute the costs to the Buyer's address and ship the cheapest way possible. Sometimes that is via "Flat Rate" shipping and sometimes it is via "Non-Flat Rate" shipping. Also, as Harry_Morant stated, the Buyer's happiness level drops when they (the customers) realize that they overpaid for shipping! Likewise, their' happiness level drops when the coin they purchased from you was not as described, so be concise in your description of the coin, any flaws, any Mint made Errors, what Variety it is and what makes it such and never be shy to talk up the coin in your description but just don't lie or give a line of B.S.! It only takes a few negative Feedbacks to totally destroy your' integrity on eBay and significantly effect the prices your' items fetch at auction.
Frank