If a PO box is out of the question, do you have any understanding (perhaps adult) friends who can keep a secret or at least not ask questions to whom the package could be mailed to instead for you to pick up later?
Of course, the problem there is that they'd guess
something was up, and might even assume you were receiving illegal drugs.
If not, you might have to try finding a store in your area that sells handcuffs and whatever else you want. Afaik, you don't have to be a certain age to buy handcuffs unless it's from a sex store.
Dare to be different... and make a difference.
To boldly go where no one in their right mind has gone before...