They're right. If they were going to be coming, they wouldn't have sent the letter, they would just have made a controlled delivery and arrested you when you got the package, so don't worry about it.
As far as ordering N+ again, I think you would be reasonably safe ordering as long as it's small quantities. Since that probably doesn't ease your concerns much, I might suggest some alternatives. This site: http://adhpage.tripod.com/coldwater.htm details how you can use cold water extraction to remove the APAP from pills, so that you're left with hydro and no apap. Otherwise, if your doctor prescribes Vicodin, then perhaps you can convince him to prescribe you a small amount of Vicoprofen (it's available in generic form now) and reduce your regular Vicodin prescription by the same amount. Most people say that a 7.5/200 vicoprofen is as effective or more effective than a 10/325 Norco.
good luck,
J.