I'm kicking myself a bit for being a lemming the moment I saw this, so here's a heads-up for the good of others:
$25 pre-owned cert is 10k points.
$15 pre-owned cert is 6k points.
Buy 3 $25 for 30k, get $75 credit.
Buy 5 $15 for 30k, get $75 credit.
You can't use the same denominations twice on the same thing, but can use different ones. If you have the points, and want to grab something pricier, don't just grab all $25 ones - mix and match the two, and you lose no value, but up the max discount up to $40.
$10 cert doesn't match up all nice and clean, but you can grab $80 worth for 32k points, which tells me you're not losing any profitability that way either. Give yourself some wriggle room.