Editied to tag @Kibble19 as well
I am not the one being ridiculous about the age because my point is that the OPs daughter's age has no bearing on what the issue is - which is the unprofessional behaviour of the delivery drivers in acting outside of their contract and intimidating a young woman who was not their customer.
Let's not be so patronising, ridiculous and insulting in pretending they are children and their brain are not developed.
I am not pretending they are children, in fact, I quite clearly said that the OP's daughter was "not a child" but it is not patronising to point out that a 20 year old will not have the same reactions to a situation as someone aged 25 and over. It is medical fact that adolescents brains are not fully developed until they are 25. The prefrontal cortex is the last part of the brain to fully develop and that is the part that is responsible for decision making and planning, and therefore reactions in the situation outlined by the OP.
There is increasing recognition that 18-25 year olds are a distinct group that need specific approaches in different spheres of life. The minimum wage, student finance, the criminal justice system, SEND provision and support for care leavers all treat under 25's (under 21 for minimum wage) differently to those who are 25 and over!
As for what 20 years olds can do - yes, of course they can be independent - as I said, I owned my own house by then and was working in a professional environment while attending uni, but the majority of 20 year olds still live at home with their parents and many are still in education, apprenticeships, training or entry level roles.
Even owning my own house at that age, having been well prepared by my parents for independence, there were still unexpected situations that came up where I needed their advice and support - there is no way that parents can prepare their children for every single unexpected and unanticipated eventuality.
Not having prepared her daughter for this particular eventuality is not some failing on the OPs part. Nor does it excuse in any way the behaviour of the delivery drivers which, as you yourself acknowledged