Questions about cards and box quality

So here's something I've found. I used to think that 300gsm was 300gsm. Not so. One manufacturer's 300gsm may be another's 250gsm or 400gsm in PERCEIVED thickness. I'm not saying some are trying trick you or that there isn't a standard, obviously. What I've found is that the coating or embossing on cards play a major role in it's snapback and perceived thickness/quality. I've ordered 300gsm cards from many sources, and every single one of them feels different. Even what you're saying here might have variability, as C2S (Kallima coating, I presume?) comes in different varieties from 8pt to 14pt. I have no idea about the "oil coating".

My advice is to ask for a free sample, and see how the quality is. If it's good enough per price for you, then I wouldn't worry about exactly how it stacks up to other manufacturers' 300gsm. If it's not the quality you want, try a higher gsm or different coating from the same manufacturer, or try the 300gsm from a different manufacturer. See if they have a sample packet. LudoFact gave me a great minibooklet with about 10 different cardstock samples all pinned together.