Has anyone ever done this? I’m selling my English saddle so that I can buy one that works for me better. But I have ZERO idea how to pack or ship a saddle, how much it costs, etc. so any advice is appreciated, haha.

I would think shipping via fedex or ups would be best. Just take it in. Get it weighed and measured. And maybe even get the packaging so when it does sale your set up. 

I shipped a western saddle by ups. It cost around $45; it was more for the size of the box than the weight. I would think about the same for an english. I had a pretty sturdy box and the saddle filled it good, I wrapped the horn to protect that during shipping. I would pack it with the foam packing cloth for an English saddle, will protect from scrapes etc. Also if selling with the irons I would wrap them too to keep from scratches during shipping if it gets jostled around. Insurance is your friend. If something happens and it's damaged they will pay for it. Pictures before, during and after packaging for insurance purposes.

I shipped a dressage saddle but I don't know how much it costs because the seller had a return label for it. The box fit it pretty well so I didn't put anything in with it, but I think shipping paper or packing peanuts would be fine if you didn't want it to move around at all. I shipped it via FedEx.

Agreed with most of above. You want the saddle to fit snug in the box. If shipping irons with it, wrap them in a non-abrasive wrapping (Bubblewrap works great!) And place them next to the underside of the saddle and possibly tape them down so they don't get tossed around druing shipping. If the saddle doesn't fit perfectly, just take nonabrasive filling (Shipping peanuts work great, or an older worn out/not used anymore english saddlepad. I wouldn't use that brown shipping paper just because I feel it could scratch the saddle if moving around too much). And yes, insurance is your best friend here.... pictures of the saddle itself before packing. A picture of it IN the box. Pic of the irons, pic of it all in the box ready to ship. pic of the box taped up with lable on it, filled out, and you want to make sure you get the tracking number for the box as well. 

Just take it to UPS/Fedex and have them pack it for you. Make sure to charge the buyer for the cost of shipping.