Level 3
I don't think there is any package for this but you could do something like this easily.
Just like a product you can let users purchase a gift certificates. Every time someone purchases a gift certificate, create a random certificate "KEY" like xxxx-xxxx-xxxx-xxxx and assign the discount amount to this key.
When someone uses the key you just discount the price from their cart and deactivate the key if the purchase is successful.