I was emailed a 15% discount reward not long ago and I clicked the redeem link. Not sure how that was processed but I was looking at some Nest devices and the discount was applied. However, I chose not to buy anything at that time and today I went to buy a Pixel 7.
I went straight to the store, not through any link, and put a Pixel 7 with free Pro Buds in the basket. Going to the basket, it had applied the 15% reward to the Buds, they weren’t free.
There was a remove-this-discount option in the basket but every time I clicked it, the page refreshed and ghe 15% reward was still there.
In the end I did order the 7 with free buds but to do this I had to logout of Google and place the order as a guest. Rather than paying £751 for the 7 + buds as a Local Guide, I got it for £599 as a guest. I would really have liked to link the order to my Google account, but I wasn’t going to pay an extra £152 for that! Best I can do now is check the order status using the guest order number 
That solved today’s problem, but I would like to know where I can manage the reward(s) in the future.