The naming of different types of cousins is referred to as “ordinals and removals”.
If you share the same grandparents, you are “first cousins”. If you share the same great grandparents, you are “second cousins”. And so on.
The children of your first cousin are your “first cousins once removed”. The grandchildren of your first cousin are “first cousins twice ...
The grandchild of your parent's sibling is your first cousin once removed. (You are also their first cousin once removed.)
Here's another attempt at a diagram, simply because this form works better for me, maybe it will for you too. The number of "removals" are the extra generations on one side only (shown in orange below), and once those extra ...
The standard family tree chart never made a lot of sense to me, so I drew the following "ladder picture", where first cousins, seconds cousins, third cousins, and so forth, are rungs on a ladder:
This visual might be easier for some folks to understand. To answer your specific question:
what about the grandchild of my parents' siblings
You are ...
If the stuff is being delivered when you say "It's for delivery" then all is well. Clearly in that case, you are speaking English and it is understood.
I'd just say "I want this stuff to get delivered to my home, please" or some variation on that. There's no standard phrase or magic word for this.
A person has a name. Although a name identifies a person, a person is not a name themself.
We would ask:
What is your name?
Who are you?
In your example, it seems the person has not heard a name properly and is asking for it to be repeated. "What" is how you would ask for a name, so "Mr what?" is correct, although some would ...
Wikipedia has some good stuff on this, e.g.
https://commons.wikimedia.org/wiki/File:European_kinship_system_en.svg for English
https://commons.wikimedia.org/wiki/File:European_kinship_system_de.svg for German.
My German wife had an awful lot of aunts (Tanten) and uncles (Onkel), whereas I have a lot of cousins. It wasn't until I saw these diagrams that I ...