The post I responded to stated 50 litres, and my comment was based on the circumstances therein. The point of the post was to put in to context the perception that it was a good deal, because it's easy to forget the actual costs of running a vehicle especially in terms of cost per mile. When you factor in the costs per mile some of these apparent savings can become uneconomic.
I'm not saying ignore all such offers, and if you are going that way for another purpose them great, but for some marginal savings it has to be wondered if they are worth it.
One case in interest, It often can make sense to come off the motorway to find fuel, rather than pay the highly inflated cost at the motorway services. But you have to judge each case on its own merits.