I doubt Amazon will match. They're only likely to match it after release, sort of like how they dropped Metroid Other M and I think a couple others the day after release (also while still offering credits).
In short, I highly doubt Amazon offers a $20 credit. Josh even said that Nintendo is holding retailers to a $10 coupon/GC MAP restriction. The fact that KMart is offering TWO of those $10 coupons is likely better than anywhere else will offer.