I completely understand......after a few shipments you will better understand how to kind of guestimate the costs. Also if you know how much it weighs...that helps. You can calculate your cost to ship on the usps website :) just a few tips for ya!
For the jacket, I would roll it up, wrap in a brown paper sack & then plastic & with tracking, under $7.00 from Mo to OK. Adrianne has some good tips. I've be doing online mailing since 2005 & used to work for USPS.