[ewoodrich]

No, that evidence would still be inadmissible. They could use "parallel construction" to obtain new and admissible evidence to implicate C in the crime, and could use the inadmissible information to launch an investigation they might otherwise not have. But the standing issue in this case is that Ulbricht cannot argue that his fourth amendment rights were violated by search and seizure of a third party's property.

39 U.S.C. § 4057 specifically requires a warrant for certain classes of mail and parcels, and therefore any evidence obtained in a warrantless search would be inadmissible.

[abcabc321321123]

Yes, that statute requires a warrant. However, the exclusionary rule is only applicable to one with a "reasonable expectation of privacy." In the hypothetical, C does not have such an expectation, and therefore C cannot challenge the admissibility of the evidence notwithstanding the statute.

[ewoodrich]

Yes, you are correct. Upon rereading your hypothetical, I realized I interpreted it incorrectly.

Of course, it doesn't mean that such evidence would be especially compelling to a judge or jury in those circumstances, but that's a separate matter.