Go to your Post Office and get a #4 Priority Mail box. A mini helmet will fit perfectly.
A mini helmet and the box together weigh less than 1 lb, so shipping should be $4.95 if I remember correctly. The Post Office sells a 1lb Priority Mail stamp.
In the box include a return label to yourself, a Priority Mail stamp and a letter requesting that they return in the same box and that you have included a return label and postage.
Hope this helps.
Michael