I work for Bank SA and this is how it works with them.
Whether you press credit, savings or cheque it still comes out of your day to day transaction account.
Hitting credit takes longer for the shop to get their money and creates a difference between your current and available balance.
Hitting savings or cheque means the shop gets their money immediately and your account balance is affected immediately, hence no difference between current and available.
Hitting the credit button as opposed to the cheque doesnt cost any more, eftpos transactions are eftpos transaction whichever options you choose. You will only incur a fee if your account charges fees for eftpos transactions.
Hope this is a little clearer. This only applies if you only have the one account linked to the card. If you have two accounts linked to the card, then you press cheque or credit for your day to day account and savings if you want to debit your savings account, but the bank will tell you which account is which on the eftpos terminal. Be careful with this one because if not set up correctly at the bank then hitting savings may not necessarily debit your savings account.
If you have more than one account linked to the card, double check with your bank which option to hit at the terminal to debit which account.
hope this clears things up a little