I've used [Monsum  and Octobat  in the past and checked out Quaderno .
What all of them have in common is that they charge a flat monthly fee, which makes it quite costly for micro/very small services, especially in the first couple of months.
What are you using?
 Monsum: https://www.monsum.com/ starts at 99 EUR/month, not recommended
 Octobat: https://octobat.com/ starts at 39 EUR/month, somewhat recommended
 Quaderno: https://www.quaderno.io/pricing/ starts at UDS 99/month, no personal experience