Why so worried? As long as you aren't shipping USPS (postal service), you are NOT breaking a law. You may be skirting a policy, but IMO just setup an account with UPS or FedEx, print your own label and drop that puppy off with anyone, preferably some kid behind the counter. If you have already labeled the box with their label, no questions at all will be asked. If anyone does ask, just tell them they are yeast samples to be evaluated. If they won't accept the package, go somewhere else.