Debiting bank account without notice on the wrong dates

I just got Fibre internet through Vox at the end of last month. I asked as many questions as I could and all were happily answered, however, the lady that I spoke to before taking the deal told me that my account will be debited at the end of every month, so I agreed and went through with it. I understood there were going to be pro-rata charges which were going to be debited only end of the month with my monthly installment because that's when the permission was given to debit. They installed my internet on the 1st or 2nd of June, sent me an invoice on the 8th and debited my account without permission or notice on the 12th. I was not prepared for this and now I have to deal with bank charges because my account went into minus. This is absolutely ridiculous because the prorata invoice doesn't even say when they going to debit you. I called the lady at Vox and she tells me they debit immediately for pro-rata charges. In my case, immediately would be the 1st which is the installation date or the 8th which is the invoice date but they did it on the 12th. I see nothing immediate about that. I did receive excellent customer service from them until this happened.