No warranty for keyboard purchased from Amazon.com

I purchased wireless Bluetooth keyboard mouse from Amazon 

my keyboard stopped working after just 4 months 

As it stopped working I took it to Dell support centre with invoice and warranty slip from Amazon.com ( click retail is seller)

Dell support centre refused to accept invoice stating that invoice from Amazon don’t show the keyboard serial so they cannot provide warranty support 

after several attempts to get invoice with serial number of Amazon they didn’t give any resolution 

I will recommend all NOT TO BUY FROM AMAZON as you will surely not get warranty because there is no way they give serial number on invoice and Dell don’t give support even with valid invoice and warranty slip as it don’t contain serial number 

What to do now ?