All you have to do is find a big enough box. Then measure the box. Round up to the nearest inch. If it measures 24 1/2", count that as 25". Now weigh the box with the saddle in it. Round up, if it weighs 34lbs, 4 oz, I put it in as 35lbs. The reason that I round up on everything is so that I know the shipping is covered and there will be no surprises when I take it to the post office.
Because shipping costs are different depending on where someone lives, I would put in your ad that if they are interested and want it shipped, they need to provide you with their zip code. Once you have the zip code, go to Postage Price Calculator
Choose "large package" and fill in all the information. It will tell you what shipping will be. FedEx has the same sort of thing if you want to compare costs.
If you want to purchase insurance or delivery confirmation, check the US Post Office website or call your local post office for the charge and include that in the shipping cost. I highly recommend that you get both.
When you pack the saddle, use lots of newspaper or light weight packing materials so that the saddle does not move around in the box and get scratched up.
Hope this covers it for you. I ship a lot of stuff and the only thing difficult about the big packages is getting through the door at the post office with your arms full.