In Krugersdorp in the late 1980s? And the police were on the case - both in terms of being tech-savy enough, and not having more pressing concerns? (1)
Let's say that it's an extra-ordinary claim and we'd like a little more weight behind it.
The police likely intercepted material delivered via regular post, and worked their way up from there. No need to know how something was ordered, as long as selling it is illegal.