Airsoft Panda are a Hong Kong company, not American, although the prices are listed in US dollars. The rules and regulations for importing were laid down six years ago and no different to buying a RIF in this country - you must be both over 18 and have a valid defence to import (are a reenactor or airsoft skirmisher with appropriate documentation to support this). It is usual to ask the exporter to put this information on the outside of the package to assist our customs agents but regardless of this expect a letter from HMRC asking you to supply details of your eligibility to import. Failure to do this will lead to destruction of your package at the very least.
I think that you may be under the age of 18?