That parts list as a whole is pretty much an entire gun minus the internals.
The letter it self is currently only a letter of detention, notifying you that your items are being held. At this time the only thing I can recommend is to go through the appropriate channels and attempt to negotiate the possible return of the receiver back to the company you bought it from with hopes you can get a refund. Hopefully with enough negotiation you will be able to have you parts released sooner or later with out much more issue.
"Its only a little bit on fire"