Do I need to stand by this commitment for the duration?

I told a friend of mine that I would watch her son (now 9 months old) one day a week. This was so she could work part time. I have since learned that she is not actually an employee. She is considered a volunteer and they are paying her in store credit. (This is an upscale children’s shop, so she isn’t volunteering her time for a cause.)

I have been doing it since the beginning of September, but I honestly do not want to continue. The child is wonderful, but babysitting is not for me. I understand now why I never did it as a job during high school.

It seems to me I have three choices:
1. Tell her I don’t want to do it anymore.
2. Tell her I don’t want to continue, but will stick with it until she finds someone else.
3. Say nothing and just keep going until the end of May.

Thoughts?