How to use HDFC Gift Plus for Online transaction (Flipkart/ eBay) ?

Hi Guys

I have HDFC Gift Coupons worth 5k.

I have tried to use them for online shopping through Flipkart and eBay. But it never works.

Does anyone know for if it even works for online transaction, and if yes how do I do that?
If no, then can I transfer the amount to account through netbanking.