Should I carry cups, straws, protein powder as inventory at a smoothie shop? or list it as expenses?

If you are a small business taxpayer, you can choose one or the other.

See https://www.irs.gov/instructions/i1040sc#en_US_2021_publink24329wd0e2345

Those are all "consumable" items in that they are used up in less than one year so do NOT use the inventory section like a regular store with products on the shelves to sell.  They are entered in the SUPPLIES area.