Just about to pay my Telstra bill and realised that this is the last payment before they introduced the dreaded PPF.
So a few sums.
Do I prepay the next bill now. Normally about $500 for 3 months. So my saving will be:
$500*1.69%=$8.45 (Amex card)
I have an offset account, so if that $500 sat there for 3 months it would earn:
$500*8.07%/12*3 = $10.09. Prepaying more than 3 months in advance is really not going to be beneficial (ie prepaying the 6 months time bill will lose me $20 for a $8 saving)
hmm, thats pretty close...what about the FF points. Living in Perth, cross country trips are the most common, so I value my points as 30,000 p=$500 (I know you can get cheaper, but as a WP X availability is better than N, so:
$500*1.5 (Amex Max Card)/30000*$500=$12.50.

So it's all about equal. I think they may have done the sums themselves to come up with a fee that doesn't reward big prepayers, but still gives FF chasers a bit of profit (pay $8 to get $12 worth of points) and offsets some of their CC costs.

To quote the old Comedy Company skit... now that's a real .......!