Changing Payment Date = Double Payment

[SFox]

Hi all, wondered if anyone could break this down for me. I just got off the phone to a customer service agent where I'd called to change my payment date by a couple of days from the 29th of the month to the 1st of the month to ensure it was always after payday.
I was told on the phone that on the 1st of March a double payment would be taken for my airtime package, then it would be normal payments from April 1st onwards.
I didn't think about it until after the call unfortunately, but why am I paying double for changing the payment date back a couple of days? Surely this would then put me into credit?

Thanks in advance.

[MI5]

@SFox 

There's no logical reason but that's the way they do it. 
They say it's because you pay a month in advance but you'd expect to be in credit anyway, as you say. 

[Marjo]

Hey @SFox , how are you doing today?

Let us know if you have any additional questions, or if you're all sorted? Feel free to pop by and ask if you have any other questions the community could help you with at any point.