non paying customers .

Yup, move toward a lien on the property.
I lived in this community for a long time,working in the parts store as well as the local garages. I know who pays and who doesent.
One iffy customer right now, i wont let car go until at least my parts and materials are paid for.
Labour, given no choice i can eat it.
Another one owes me a little, and if i never get paid that means they wont come back,fine with me.

Good luck, it sucks when your hard work goes un-rewarded.