I think post-dated basically means "not today's date". If you send a cheque with a date later than the due date they won't be able to cash it, so you'll be in default and have to pay charges.
What theyr'e asking for, I imagine, is that you send cheques they can bank straight away so they don't havo to arrange them in piles according to when they can be banked. I can see their point.
Have you considred setting up a direct debit to pay your bills on the due date each month? It saves a lot of hassle.