Did you have payment terms written down before starting the work? That's honestly the first thing - if the agreement was clear about when payment was due, a simple "just following up on the invoice from [date], let me know if you need anything else from me" is perfectly professional and not desperate at all. Five days isn't even that long for some people to circle back, but if they're ghosting specifically on payment after being responsive during the project, it might be worth a quick check-in about whether there's an issue with the files or if something came up on their end. One thing I've learned is that people sometimes go silent not because they're dodging you, but because they're swamped or waiting on their own approvals - a straightforward, deadline-focused follow-up ("just want to make sure everything's good; let me know by [date] if you need revisions") keeps things moving without sounding needy.