TLDR; Gas (petrol) stations pumping less than you paid for is a problem in some parts of Mexico (they're mostly full service). These guys are trying to market a device to measure the amount actually pumped into your vehicle.
This site downloaded a 1.2KB PDF to my phone! I suspect they have maladverts. It doesn't happen the second time.
TLDR; Gas (petrol) stations pumping less than you paid for is a problem in some parts of Mexico (they're mostly full service). These guys are trying to market a device to measure the amount actually pumped into your vehicle.