You need to:
1. be logged into the Cashrewards website first
2. click on the eBay (or any other store) promo
3. click on mustard-coloured "Go to Store" button on the right pane
4. wait patiently for this link to load and store cookies on your device



5. your browser would then end up on the eBay page - you should see that the link is an odd code like this:



The code after the grey "?clk_rvr_ID=" should represent your individual account details. Don't bother using the one in my pic unless you want your cashreward refund to go to me

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I've used cashrewards a few times on different websites and always got a refund some time later. The beauty about the eBay discount is that eBay processes the payment as a full payment and Cashrewards pays the refund based on the processes amount.
i.e. eBay discount of 15%: I paid 85% and eBay paid 15%, and Cashrewards' refund was 2.5% * (my 85% + eBay's 15%).. yayyy negligible profit..