Once the files are delivered, your main bargaining chip is gone. The issue is common enough among freelancers that invoicing and contracting platforms make it a recurring piece of advice: ask for a deposit up front, then the balance on delivery, never the other way around.
The principle: the balance before the file, not after
The rule that changes everything: never hand over the final deliverable until the balance is paid. It isn't a lack of trust in the client, it's standard practice in creative fields (photo, video, design) that keeps the relationship from souring over an unpaid invoice.
In practice, the problem is that most transfer tools can't enforce this rule: once the link is sent, the file is accessible, paid or not.
Lock the delivery, literally
That's exactly what Paylock does on xFer: you set up your delivery with an amount due, and the client sees their gallery or their teaser, but the download of the final files stays locked until payment goes through. No awkward chasing to do, the system enforces the rule for you.
The client keeps a good experience: they see their project is ready, understand what they're getting, and unlock access themselves by paying. You no longer have to chase a balance once the work is handed over.
Frame it in the quote, not at delivery time
Set the rule out before you start, not at delivery: "the final delivery unlocks once the balance is paid" belongs clearly in the quote or the contract, never as a surprise. A client told up front never reads it as distrust.
The same goes for staged payments on bigger projects: the more your payment milestones are tied to concrete deliverables, the less an unpaid invoice can pile up across the whole project.
In short
The balance is paid before the client has the final file, not after, and it's framed from the quote onward. With a delivery that locks access until payment is made, you no longer have to choose between nagging and losing your balance.