You finish the work. You send the invoice. Then you're checking the bank manually to find out if you've actually been paid, and sometimes sending a chaser you didn't even need to send.
The cost isn't the client being difficult, most invoices do get paid eventually. It's the time spent not knowing. And it compounds: an invoice you're not actively watching this month is just as easy to lose track of next month, so the pile of 'have I actually been paid for that' quietly grows across the year.
Watch for the payment landing, work out what it means, and have the next step ready, a thank you or a polite chase, without you checking manually. Nothing moves money, nothing sends without you seeing it first.
Imagine never wondering if you've been paid. The moment something lands, or falls overdue, it's already flagged, so unpaid work stops quietly fading into 'I'll deal with that eventually.'
There's an example of this here.
See exactly how the watching and the follow-up work, step by step.
See the example