outstanding charges

sanral is taking us for a ride here. I have contacted them regarding my invoice which says that i owe as a non payer. according to them my account was deplicted and hence even when i loaded money afterwards it meant the outstanding amount still remained unpaid. i get their letter more than a month later and now have to pay twice the amount stated there. they tell me it's not their fault post office took forever to deliever the letter, question is: What happened to them sending registered letters, becaus then they woud know I did not receive the letter? so it seems sanral is chowing the very people who are paying for the stupid tolls now. they are enriching themselves with our kids' money. why wait two months then post an invoice? and why didnt they send it via email like the others I receive every month? I got an sms notifications, why did they not send that via sms still? whose fault is it if they letters dont reach us?