You don't pay Giant $77 because I paid Giant enough points for it--you pay me $77 instead. I have more than enough points to cover the entire purchase of the gas (the receipt will say $0.00 and deduct the points (see under Gas Savings). I don't carry change so Venmo is preferable, probably safer, and this will avoid any physical exchanges. There are better ways for me to get more value out of the points but I feel this is probably the simplest/least sketchy way to do it for both parties so I'm willing to "sell my points a discount".
Buyer drives to pump, I come up to scan Rewards card, and pump will say $0.00. Buyer fills, pump automatically stops at 25 gallons because of redemption. Buyer pays in cash/Venmo and we leave. I understand not everyone has gas tanks to fill remaining gas after their vehicle and I can work something out (minimum purchase will be 15 gallons then--worst case scenario I can come up with two of my gas tanks for myself (I have a couple of these on loan for a deposit). In the past I have had a buyer bring 2 vehicles but a particular gas attendant was not impressed.