USPS recently notified us of a policy regarding liquids being shipped.
"All liquids must be shipped in a container sealed with a screw-on top turned at least one and a half times, with a clip or by soldering. If the liquid is in a breakable container with a capacity of more than 4 fluid ounces, cushion it with material that can absorb the liquid in case the container breaks. Place the breakable container surrounded by cushioning material inside a sealed, leakproof container, and place that container within an outer mailing container."
To sum this up, our bottles would need to be put in to a sealed bag, then surrounded by some sort of materials that could absorb the liquid, then packed in to a box that is then wrapped again, and finally put in to a 2nd box.
This would added $7 - $40 per box to an order making USPS no longer an cost effective option.
UPS has stepped in to fill this void by providing us with their new flat rate service.