FNB charges way ubnormal

I was using a pay as you use account, then got a call from FNB to upgrade to another account with the promise of ebucks. Just this month end I realised they charged me R945 for a bounced debit order of R3900. Which said item paid no funds. I was shocked on how they calculate their interest on those bounced debit orders , especially within 24 hours. I was not informed of that when I was asked to upgrade . I need help for refund and also to downgrade the account to pay as you use.