It seems that 20% cashback is for all categories
Special Terms
Cashback is ineligible on purchases made via the guest checkout option. Ensure you are logged in to your Groupon account prior to purchase.
Cashback is calculated on amount paid after use of any Groupon credit.
Cashback is ineligible on purchase of gift cards.
Cashback may be ineligible if using codes not listed on Cashrewards.
You must return and click through from Cashrewards every time you make a new transaction/purchase.
Bought discounted $14 Lunch voucher for $11.90 with code BOXING30 (=15% off)
FREE AFTER:
16% cashback = $1.90
Plus $10 cashback on $10 spend (targeted)