I've read through this and it sounds fairly simple. However, do people only sell the AUs on cards they don't use? I have a Chase card that is 2.5 years old with a $25k credit line and a Elan Financial Card that is 2 years old with a $25k credit line. I use both on a monthly basis and rarely spend more than $4k total per month on both cards (I put everything on these cards including business expenses). Can I use these cards or will that mess with my regular usage? I also have a 7 year old Discover Card with a $7k credit line that I rarely use. It seems like that is the most likely to use for this, but I'm still a bit confused as to why using cards that you don't use regularly are the ones to go with.
The whole point of the exercise is to help raise the AU's credit score. The tradeline company wants you to have some utilization so that the card reports on the AU's credit report, but not so much utilization that it doesn't help the AU score as much as it could. I believe the guideline/requirement is less than 10% utilization.
Personally I have a bunch of cards, so using a non-piggybacking card is trivial. When I have AU orders, I charge $2.50 to the card each month and pay it off after the statement closes. That makes the utilization high enough to where it reports on the AU's credit report, but very low utilization.
$4K on a $25K card exceeds the 10% recommendation, so I would either move your spending to non-piggybacking cards, or don't use those cards for piggybacking if you'd rather use them as daily spenders. From what I've seen, I think your younger two cards with higher limits would be more appealing to AUs than your Discover card, but you might get sales on all three.