There are a bunch of $.99, $1.49, $1.99 fillers on the PSN store that'll bring your order above $50 to qualify for the $10 back. Or you could grab a movie or a $5.99 ps1 classic, whatever floats your boat
These two promotions ($10 back and 15 months of ps+) overlap until 3/4
Nice suggestion about ps1 classics. I'm hopeful that at least digital ps1 purchases will be playable on the ps4.