Extra charges on interest

Bmi literally took things via PayJustNow in December... they charged me interest four times before their next payment...

What annoys me is that I'm not getting help on getting understanding or my money back and they deducted on the 28th of January and say last payment is on the 31st of January...

So basically using PayJustNow is pointless if i have to pay for something on the same month... it literally defeats the purpose