Thanks very much, NigelH and Mouser, for your replies. I think NigelH may have provided a good solution to the problem I was having. I was trying to have a three-level arrangement. The top level--the Category--would be the store (e.g., Safeway). The next level, which I suppose would have to be the Label--would be the kind of item I wanted, for example Produce. And under Produce would be individual items such as grapes, tomatoes, butternut squash, etc. What I think NigelH has proposed combines levels two and three into a single level, the Label, which would combine a level-two designation AND a level three: produce-tomatoes, produce-butternut squash, etc. Actually, I think I like Nigel's replacing Produce with the more specific terms like "fruit" and "veggies". And if I list my items in alphabetical order, at least I'd still have all the veggies listed together, all the fruits listed together, etc., even though the veggies wouldn't be listed near the fruits.
Mouser, I don't think screen size was an issue here. I was able to see everything (indeed, my phone, an LG Stylo 4, is toward the larger end of the phone spectrum). My problem was partly terminology, but mostly I was trying to have a three-level arrangement, not realizing that Checklist was designed for just two levels. But NigelH's suggestion to have the Label combine two levels into one should give me what I need. My one concern is that, especially since the Label font is larger than the Category font, there may not be enough room to include, say, "veggies-butternut squash". I wonder whether you could make possible a choice of font sizes? Or, if not, at least make the Label font smaller than it currently is?
Again, many thanks to both of you!