Discovery Health: Discovered Debit Order Debacle

Discovery Health is out of touch with its customers. My debit order is currently deducted on the 1st of the month. My salary pay date is changing and I attempted to change the date of the debit order.

You only have a few dates available to choose from - 1st, 10th, 15th and 25th of the month. I elected to change to the 10th of the month. I was then advised that my debit order will be deducted on 01/03 (current debit order) and again on 10/03 (for April). Two deductions in the same month - I queried why they don't deduct a pro rata rate for the two outstanding days and was told an unequivocable "NO" - this is not negotiable, this is their company policy.

Please explain how doubling a monthly subscription is fair and acceptable.