If you can lay down a solder bead, you can do it yourself for about $25 bucks total. Nickel cover will run you about $12 a piece. Then all you need is a little parifin wax melted in an old pot. Press the covers on tight, lower the pups into the melted parifin wax (not too hot, just clear liquid) and wait for the air bubbles to stop. Take them out with the baseplate facing up, wait for the wax to turn milky. Clamp the covers down tight with a padded clamp, solder the cover one bead in the center on each side, and clean up.
I would imaging that sending them into duncan would take at least 3 weeks, and outside of the cost of covers, not sure what the charge for potting is....but it can't be much.