Small task In WordPress Gravity Forms

I have developed a website which have Gravity forms and from which i am accepting PAYPAL. I also have added Coupons plugin to add coupons functionality and it is working as expected. Problem: What i want to have is coupons should be automatically generated and mailed to the buyer. Coupons discount is the same for all coupons. And after the buyer receives the coupon, he can use it in any other product from website(This part is also done to match the coupon and give the discount). I just want to create random coupon after successful payment from paypal and mail that coupon code to buyer. Simple task not more then the 3-4 hours for experienced. P.S. I am also a developer and tried all the options, there is no plugin to do so, thus if you have done already or know how to do then only bid. Total Project Budget: 1000 - 1500 INR ~ $20-$25 USD