Hi, crates.
If you have highlighted the correct 10 cells, then the number of items should be obvious from just looking at the grid. The one-word definition of the phrase revealed by the discarded letters (which consists of two two-letter words) is given as a synonym (described as informal) by Chambers in its entry for the phrase.
Many people who use the express till ("up to 10 items", for example) in a shop interpret the restriction rather loosely, and I suspect that is why the setter has included the question mark in the puzzle's title.