I have bought and sold at least Five pairs of speakers over the past eight years. Never a problem. (UPS, FedEx, DHL.)
I suggest the purchase of the multi colored carpet padding used by installers.(works great!), another good choice is to use foam padding used by upholsterers or dense padding sold in fabric stores (Jo-Ann Fabrics etc). The above mentioned styrofoam with reinforcement is also a very good idea as is strengthening the container/box.
You can by from suppliers the materials you need or consult a packaging house or other shipping service.
Is there a small business near you that frequently ships items that are similar in size and weight to your speakers?
They may be willing to assist you for a small fee, or offer suggestions.
Always ship with insurance and take pictures of your packaging process and use the time stamp. Best of Luck!!!