Find other sellers in your location, agree to ship your items together under either one of your businesses'. This is done by contacting USPS or UPS, DHL, FedEx, etc and saying "Hey, I ship 640 items a month that weigh less than 14 ounces, we would like to receive lower rates or we will ship carriers because they are offering me ___% off"
Don't quote me, but I THINK if you set up an account with reserve funds with USPS.com and you ship 400 packages a month that qualify for First Class Mail, you can ship items that are 1-6 ounces for like $0.87 cents with tracking. Its been a couple years since I first started negotiating shipping rates. There WERE other little things that they gave discounts for, such as being an eBay seller but I believe they might have discontinued that.