If you buy directly from happy cards they're usually full price. They take promo codes but I haven't been able to find one.
I get them from my credit card promos. Recent ones I've encountered:
Amex Offers - Get $15 back when you spend $100 at Happy Cards
Chase - Redeem your rewards for discounted happy cards. 0-10% discount
Wells Fargo - Redeem your rewards for discounted happy cards. 0-15%
Discover - Redeem your rewards for discounted happy cards. 10-20%
I've found the discounts change wildly. When they bump up to the high end 15-20% they usually only last a couple of days maybe a week. But they seem to happen frequently approximately once every month
Then there's the store ones.
Staples, Lowes,
Amazon, others will have discounts from time to time.
Giftcardmall just had a 15% bonus expire yesterday. Spend $50, get an extra 15% of value.
Right now..
There's also Target spend $50 on a happy card get a $5 target gift card. But this isn't attractive as the others.