I would make them to commit to one specific form of pay. They either pay monthly every time or weekly every time, but there is no going back and forth. As for reminders, I would stop reminding and enforce late fees and also have a 3 strikes, you are out policy. You shouldn't have to chase your check. If it was an occasional thing, I might text a reminder, but habitually being late is not ok and will cost them more in the end.

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