It would certainly be interesting, however in the absence of hard data perhaps some anecdotal evidence is better than no information:

Case 1 ------ 10 years x 280 deliveries per year x 2 deliveries per day (pick up + drop) = 5600 deliveries.

Number of mistakes = 0

That puts the error rate at < 1/5600 (or approximately 0.0002).

Case 2 ------ 4 years x 280 deliveries per year x 2 deliveries per day (pick up + drop) = 2240

Number of mistakes = 0

That puts the error rate at < 1/2240 (or approximately 0.0004).

Both cases together -------------------

Error rate < 0.00012