I am so disappointed because Miway broke the agreement in terms of monthly premiums

I am so disappointed because Miway broke the agreement in terms of monthly premiums. I joined in September and we had a written agreement that monthly premiums will be deducted on the 15th if each month. Boom today 12 October there is a debit order from Miway and that has caused negative balance on my account. If talk to their boot the response was that they Apologize. My fussy is who is responsible for charges of insufficient funds on my bank since they decided to bridge the agreement? I want them to transfer back my money and pay those overdraft charges. What a disappointment