Be fortunate that this occured with a cold-hammer forged HK barrel. Other barrels and you might have found out in a much more violent manner. HK specifically tests the MP5 barrel to "bulge" and not split or fragment. Because of HK's manufacturing processes for the steel itself and their CHF process, the uniformity of its composition allows the barrel to bulge uniformly. As such, it is claimed that an MP5 will loose no accuracy (distance unknown) as long as the bulge is greater than 4mm from the muzzle.
Ok, enough with the history lesson. While it is not my place to assure you of mailing integrity, I find USPS Prioroty Mail with adequate insurance and dlivery confirmation has always served me well. Optionally, USPS Registerd Mail is one of the safest ways to ship a package and is my preferred method of shipping items greater than $5000 in value.
HKParts.net sells the RCM cold-hammer forged barrels. While they are CHF, they have not been tested like the German CHF barrels have been (bulge test). They also have been nitride treated which makes them a bit more resistance to wear, but can also increase its brittleness. What does this all mean? Your guess is as good as mine. All I can say is they are are excellent barrels. I have installed dozens of them.
Given that your gun is an HK94, you may wish to consider a factory German MP5 barrel. They can be found from time to time and are typically only about $100 or so more than a new RCM barrel.