I'm assuming when you say 3D design, you using either CAD or modeling software? If yes, you could consider selling STLs as well as the finished product. Giving away, or heavily discounting, STLs for an alpha/beta run, would eliminate your shipping and material cost for testing. You would still be able to sell physical parts, as well as produce parts that are nicer than your average hobbyist will produce (for example, print molds and cast the planter in a more traditional material).
As far as where to post, I would try [email protected]
This all assumes you're working in software that can generate STLs.