I’d avoid sending another vague “just checking in” note. At this point, I’d use a short sequence: resend the invoice with the amount/due date/payment link, ask directly whether there’s any issue with the work or invoice, then set a clear follow-up date.
Something like: “Hi [Name], just following up on Invoice #[X] for $1,500, due [date]. Could you confirm when payment will be made? If there’s an issue with the invoice or deliverable, please let me know so we can resolve it.”
The key is to stay calm but make the next action explicit.