Funny, I usually blame the courier first. Some have an established track record of delaying deliveries, or claiming that they tried to deliver but nobody was at home. (I call the latter the "Finnish post office" strategy.) Repeat offenders get punished by not having business in the future. Eventually.

However, you are right from a customer's perspective. A third party cock-up is not an excuse - it may not be your fault, but it still is your problem.