Or if you want to be extra, extra specific, get them to write this on the side of the box in bold:
"These toys are being imported for use the acting out of military or law enforcement scenarios for the purposes of recreation - AKA "skirmishing", as outlined in the Skirmishers Defense in Section 36 of the Violent Crime Reduction Act.
The recipient, **Your Name**, is a registered member of the United Kingdom Airsoft Retailer’s Association (UKARA), membership number **Your UKARA Number*"