Chasing invoices is awkward.
So let a machine do it, politely, on schedule.

1. 1
   Invoice sent

   With a pay link, from your accounting tool.

2. 2
   3 days before due

   A friendly heads-up email, never a threat.

3. 3
   Due date

   Email with the amount and the pay link.

4. 4
   Late: SMS, then a call task

   Day 7 a short SMS; day 14 a person calls.

5. 5
   Paid: it stops

   The payment closes the sequence on its own.

The schedule is an example; you choose the days, the channels and the tone. Every reminder checks the invoice is still unpaid before it goes out.