Fact is you can get $200 for $195, making this %2.5 back.
Add 0.5% from GCR.
Add the $0.03 per liter. We'll count this as a 2%
Add the percentage you will get on your CC for your online purchase (normally somewhere between 0.5% and 2%)
So, all in all, one could reasonable expect to get a 5.5% back on purchase after everything is accounted for. In most cases, this is still better than with the credit card alone.