I am pretty sure you can (we're talking in-store, right?).
One more thing (because that $15 off $50+ coupon is probably already expired), they have a new 30% off coupon (probably as good as the $15 off $50+ coupon) valid until 13 december 2012. The code is HOLIDAY and you can use it online (probably multiple times?!).
And they have a $15 off $75 coupon too (for in-store use). And more coupons here: http://www.printablecouponsdownload.com/old-navy-printable-coupons/
Hope it helps! :-)