All of the above answers are good..... if your gonna fly rocket for awhile ya need to know about this site, [almost] ANY thing you want can be found here:
https://www.mcmaster.com/#push-rivets/=kypcl8
As stated Rivets are rivets. [for holding things together semi-permanently, they can be removed & re-used for many, many flights]
Shear pins are shear pins. [used once, they break/shear as designed. new ones used for every flight]
I haven't got into shearpins yet as I am still waiting for my NC to arrive.
You don't mix the two.
They come in packs of 50 or 100, by far the cheapest way to buy.
Edit: some folks use RIVETS to hold a nose cone on a payload, that is NOT for dual-deploy. Such as carrying experiments or an altimeter that is read altitude only. In this case the rocket uses motor eject for parachute deployment. They want a solid hold, but yet removable & re-usable. Shear pins are not for this[generally, got to cover my butt here]
I think this is where your confusion comes from.
For our use here.... it is:
Dual Deployment..........................
Shear pins in NC
Rivets or screws to hold av-bay on payload.
Forget everything else you are reading..... call/write them for their explanation, we can't speak for them. I don't get Apogee's explanation nor do I care. That's not the way any of us I know use them. For transportation, common sense tells ya to leave them in & just pull the upper-half apart with av-bay still attached....otherwise all your gear can fall out.
Like I say many times......"there are many ways to do this, none right or wrong" I just think the description here is one of the most no-nonsense...common sense approaches.
Judging by how many folks use it, that speaks for itself.
These are the ones I use below.