Incomprehensible Interest and Commission Calcs

I agreed on arrangements to pay my FNB accounts via this Legal Firm I have already made 2 payments . I was shocked when requesting the statement that the fees seems to be added arbitrarily and it seems the account is on standstill due to this. I have have no problem with Helen who gave me the statement promptly. All I want is an explanation on which balance do they calculate the 10% interest and on which date , how many times per month. I also would like to know how they calculate the commission and on which date. Without this information it is difficult to plan ahead and it seems like the amounts are being charged willy nilly.