(see contracts/GaslessNFT.sol)
totalGas = 164572
gasStart = 140750
gasPrice = 1.000000008
cashback = 105844000846752
cashback = (gasStart + 21000 - gasEnd) * gasPrice
gasEnd = gasStart + 21000 - 105844 = 55906
gasUsed' = 140750 + 21000 - 55906 = 105844
gasUsed = 116415
const delta = 116415 - 105844 = 10571