Basically you pay $15 for a $60 10 cent price saving card. That's 7.5 cents/L saving. But you will have to pay $300 upfront. If you have a gas cash back card or PCMC, that's not a good deal.
A couple months ago, ebay has ESSO GC sale $100 for $95 with 10 cents/L price saving card for 100L for free. That was a good deal.