Hmm, that's the workflow already. If you're shipping at least one Priority package, the postman will pick up your packages for free. They'll deliver boxes to your door for free, too.
Don't mean to take anything away from sold (amazing product) but we are starting a company that takes care of the shipping, for anything. Check it out if interested http://shyp.co
It's a pain if you don't have boxes and tape and whatever, that's true -- looks like your service handles that for you which is pretty sweet.
Another huge thing people don't realize is box size. In most cases you can save up to 50% off the shipping price if you can fit your item in the tightest box possible. There are only a few sizes USPS provides for free using priority mail.
We have about 20 different box sizes to take advantage of this.