UNCISG - Offer/Invitation

Hello, I saw in the UNCISG video that goods in the supermarket and shop windows are invitations, not offers, and I kind of have a hard time wrapping my head around this. The explication in the video is that, in supermarket for example, the price listed next to an item is not necessarily the final price (the example was cornflakes for 20c, while at counter they were 2 dollars). I can understand this being true in a shop where negotiations are happening while buying a product, but not in a supermarket where prices are fixed (I have never been in the situation where the price at the counter differed from the one on shelf). I know this is just an example and buying items for personal use, but I'm trying to understand the point through this. I can imagine the situation where you, as a company, buy items from supermarket and depending on the volume you get different prices (x$ for yQ, w$ for zQ) but to me these also seem to be offers, not invitations, because you can accept without needing to make an offer. At best you can make a counteroffer. Can you maybe help me with another example or expand this one more? Thank you, Cristi