Accounts in arrears for successful transactions

I am so furious that my account is now in arrears for transactions that went through successfully over a week ago, which are now only appearing on my statement. Surely they should have declined if there weren't enough funds available. I rely on in contact and email notifications. I cant calculate every cent that goes off my account. The debit should decline if there are not funds, regardless of whether the supplier or service provider has processed their banking. If the card machine successfully processes my debit then it should be deducted from my total balance or recorded as such. I constantly view my online banking profile and if the balance is in positive then I have money to spend. FNB cant all of a sudden deduct the amounts a week later and say, oops you don't have enough funds you now owe us money. My account is a debit card not a credit card. I feel this is extremely inconvenient especially for the fact that I am not wreckless with MY money. I sincerely feel this is not my problem and extremely unfair to treat me this way. This has never happened before. I am always able to see my balance vs my available balance and this is how I manage my spend.