She wants to pay me on a different day than everyone else -- should I let her?

[Artsand crafts]

I would send her a letter and add that late payment fees will apply when payment is not done the scheduled days. By paying you at the middle and at the end of the month she is paying you less per year than the other parents (if you are not adjusting her rate). I would be more concern about that than she not following your rules. If you charge for example $350 per week, assuming that you are paid the whole year (52 weeks or 26 bi-weekly payments) you are paid $9,100 ($350 by 26 bi-weekly payments). If you are paid twice per month as this mom is doing you are getting $8,400 ($350 by 2 times per month by 12 months). She is paying $700 less per year than the others. She is skipping 2 bi-weekly payments per year!

Here parents paid on the same schedule, not exceptions. Since I get better organized this way.