You communicate clearly, politely to your primary customer contact. You explain that the commercial arrangement is you provide service, their side of the deal is the pay on time every time and you never need to chase them.

You explain kindly that this is the only work arrangement that works for you, and that your company policy is to stop providing service if there’s an outstanding bill.

You explain kindly and patiently that it’s not practical for you to work with companies that can’t pay the bills and you’ll end the relationship if there’s three times it happens.

Then you stick to your word absolutely. If you waver, fold, let them break the terms, then none of this was worth anything.

Be willing to drop customers who don’t pay on time.

You’ll find mostly they train up pretty good. The rest ? Don’t stay in an abusive relationship.

this does NOT need to be explained. They have accounts receivable too. They know how this works. Put the process in the contract and follow the process.