If you are outside and using credit cards, my advise will be to take a photo and store into a folder in the gallery.

Later, let it sync to Google Photo or iCloud and arrange from there later to tally with the bills.

Usually i use a basic Excel spreadsheet to log all my sales transactions and costs each month. It has different tabs to break down sales by product category, expenses by type, and I can see my net profit/loss. I know I should eventually get more complex accounting software as the business grows. For now as a small operation, Excel works fine to monitor the basics.