Which is why you need airtight service agreements, and then the willingness to follow up on them. Obligatory video, "Fuck You, Pay Me": [https://www.youtube.com/watch?v=jVkLVRt6c1U]
I only ever had two customers (out of > 200) not pay in about 15 years. One got a lawyer letter and paid up. The other (new customer, and one I was warned to be wary of) I actually dragged to district court to receive a judgement. They then still wouldn't pay. I figured there was no one there I'd ever see again in my career... so I sent the sheriff. They wired payment within hours.