This app was mentioned in 4 comments, with an average of 2.00 upvotes
I either use my phone calculator or a simple counter app (for Android, Click Counter Free). I round to whole dollar amounts to keep the math easy. For my shopping list, I use a checklist in Google Keep: when I buy the item, I check it off, and when I run out, I uncheck it and it goes back into my list.
Click Counter Free link:https://play.google.com/store/apps/details?id=digitalfish.counter&hl=en