Payment dispute

I have purchased a dress on the 13/02/2023 at poetry via PayJust Now app.with the app you will pay whatever you purchased with 3 installments with no interest.On the 24/02/2023 I have received an sms that they are going to debit my 2nd installment on the 25th which is before month end.I have tried to call them sothat they can move the debit date to month end with no luck .No one explained to me that how it works.I have send an email to explain to them that they must move the debit to month end and the reply I got was that they cannot move it and they added interest. If it was explain I wouldnt have bought from them.Now I need to pay more money for something that was not properly explain I think this is not fair. Nyalleng