Reaping people off

I'm so puzzled I get a call today from woolworths saying that I missed an installment, which is a lie I paid January 26th and I paid for February on the 9th, it's not even month end for March, this lady tells me that yes you paid on the 9th but it's considered as for January, the cut off is the 11th so if you pay before that its for the previous month, now my question to her is why weren't we informed when we joined woolworths, and she says ow I'm so sorry for that but still this month you must pay 2 installments, that's ****, so now woolworths is reaping me, I'm not going to pay for 2 installment