Does the "Update Items" button update all the prices in the shop at once, or does it just update the items on that individual page?

one page at a time. If you leave a page without updating whether your new data is lost depends on your browser, but usually it is lost. If you want to update just one item press return at the end of your entry, otherwise you can mouse/tab your way down the page.

Cool, thank you so much!! I would’ve never guessed hitting enter on an item would update it individually. (Apparently my browser loves me cuz I’ve accidentally clicked away and it had saved my entered prices, lol)