Green Man Gaming takes 10% off PlayStation Store gift cards via coupon code "PSNCREDIT10". (You must sign in to redeem the discount, and you'll receive a code to redeem the credit.) Generally speaking, that's a rare discount for PS Store credit; most retailers almost always charge face value. Sale ends August 31. The deals, with prices after coupon:
$10 PlayStation Store Gift Card for $9 ($1 off)
$20 PlayStation Store Gift Card for $18 ($1 low)
$50 PlayStation Store Gift Card for $45 ($3 low)
Note: This coupon appears to work on multiple quantities of these gift cards.