Hi Keith
By way of introduction, I am now retired but was recently the Purchasing Manager within the Royal Mail for a multitude of contracts.
One of the contracts that I had to set up was the measurement of service delivery performance for the Royal Mail by independent auditors.
In brief, thousands of letters and parcels were posted at different places and the times of despatched were noted as well as the time of arrival. Also late deliveries and losses were recorded. Other large delivery companies did the same throughout Europe. All this was done by auditing agencies who were paid to conduct the tests and part of the deal was that they carried out the tests over several months in utmost secrecy.
The good news was that for year after year, the RM came out right at the top of the pile compared to competitors mainly because it has a better infra structure than the competition.
Delays will occur, that is inevitable but I would still recommend using the RM because to my knowledge,it is still the most reliable.
Bearing in mind the value of goods that you deal with, do a test yourself for your own piece of mind, post an empty parcel to a few friends and see what the results are.
Also the percent chance of a non delivery are so small, there are not enough noughts on your calculator to work out the figure.
Regards
Mick