Give Customers A Little Credit
The problem with these subject lines is they're just offers. And every website they've subscribed to is making them an offer that's easy to refuse.
But take a look at this subject line:
You Have a $10 Credit With 1-800-Headsets
This is psychologically powerful. Of course, unless a credit is issued as part of a refund (credit note) or a gift card, a "credit" is just a clever name for a discount. (In this case, the credit is offered because of a previous purchase - i.e. a X-off-next-purchase coupon). But the word credit suggests "we have $10 of your money on our books, come and redeem the goods that are rightfully yours." This may be more motivational than the played-out discount offer.
Definitely worth testing.