Shell Citi credit card
Once through the introductary period it's 3p rebate per £1 spent on Shell fuel and oil, and 1p per £1 on any purchase anywhere (providing you've bought Shell fuel at least once during that charging period (month).
Rebate applied directly to your statement. So if a charging period (month) you spend such that you're entitle to a £8.82 rebate, the £8.82 will be subtracted from the amount outstanding.
Shell Drivers Club card
The rebate is 1p per litre of Shell fuel and oil.
I opted for a quarterly rebate which previously arrived in the form of vouchers in increments of £2.50 (500 points IIRC) - not discount vouchers, like gift vouchers really.
This month they've switched me to an automatic credit to my Shell Citi credit card, presumably because the transaction cost to them is cheaper. Suits me, as long as I get my rebate.