Weigh it and then go to usps.com, use the calculator and look at the media rate. That's what you can use for books, usually the cheapest way to ship books or other paper materials.
One other piece of advice, if you think your item will be attractive to a number of people, start your bidding below 100 credits. With any type of auction, the lower the starting bid the most people who usually are interested. With a low starting bid, people can also put in a low "watch bid" to get it on their list, and then come back and bid more later if they decide they want it.
The risk is that your item will sell for under 100 credits, but if you have something you KNOW will go at least that high, start at like 11 or 13 credits. You'll get a lot of watch bids, and then people will wonder what is so special about the item that it is getting a lot of bids!
that is good advice thank you, i try to get on here twice a day between work, and other things i have going on, I know I need to keep up with answering comments, checking my watch list to see if i won or need to bid higher with what small amount of credits i do have, and check on the stuff I listed