Does everything produced by The LEGO Group count as "LEGO pieces"? I'm wondering if things like LEGO store plastic bags, LEGO boxes, instruction booklets, packaging etc., count as "LEGO pieces" if I incorporate them into a build for the purposes of posting as a purist LEGO MOC?
There is no official answer on this, as “purist” LEGO is a consensus, not a definition. That said, there are several examples of pieces LEGO doesn’t classify as a part that are used in MOCs, such as:


The pin used to keep bundles of 4 train tracks together.





This Technic hub piece was never originally intended for standalone use (it was part of a pin joiner) but has been used by MOCcers due to having antistuds on both sides (within a 1x1 brick footprint). It was later used as a standalone piece in official sets before being replaced with a new design.





The iconic brick separator is not intended as a regular LEGO piece, but has a whole class of MOCs based on it. I have seen it used as railway station roofs, spaceship wings etc.




Net, I think the general consensus is that if it has been produced by LEGO and you can find a way to incorporate it into a MOC, your creation is still considered “purist”. At the least it gives you good arguments for any debate that might arise. 

Curious to see how you will incorporate shopping bags in a MOC ... I can imagine one being used in a rather large scale MOC where you utilize the LEGO logo on it as part of a display.