google sheets - I have one sheet with date, # hours, notes - I tally this up every couple weeks or month whatever the billing interval is, copy that number into another sheet that looks like an invoice, print to pdf and send that to my clients via email.

Most of them pay via electronic means into my business account, but I've taken paper checks from usps mail as well. I have a small number of clients - 1-3 at a time at max, I can do this in ~5 minutes per month for all of them.

Google "google sheets invoice template" -- I use an early variant of this one:

https://docs.google.com/spreadsheets/d/1TzvUubN-M9q4ATPCuzuK...