LC can be mysterious
IME not all same-day Amex transactions count and I stopped doing multiple low value transactions as so few would register (at one stage I thought there was a $30 threshold but have since had lower ones register) and it would be easy to be disqualified under the T&Cs covering gaming. I tried using different types of checkout in the same store with inconsistent results.
On the plus side, if you do make LC, all your transactions that day seem to count. This could result in 2+ people being LC on a particular day.
At one stage both I and mrs andye were showing as LC at our local Coles but looking back this was after I had passed 10k for the year so was no longer earning.
If you have more than one Coles nearby (I have 4) I'd target the least-impressive one as you will have less competition