Send Stage 1 a few days after the due date passes, friendly, assumes it was an oversight. If there's no response after another week or two, send Stage 2, which is firmer and references the original terms. Stage 3 is a final notice before you escalate to a formal dispute bundle or collections. If the amount is significant and time-sensitive, invoice factoring is worth understanding as a last-resort option, though it comes at a real cost.