If their value proposition is true (the customers would otherwise not have come to you), then it is 8% off sale that you otherwise would not have received. Paying typical card fees, but receiving no sale is a far worse proposition.
If every single order you get through it is an order that you definitely would have never received without the system, then it's a good deal. Other than that, it's maybe good once, but when customers start using it on a repeat basis, there's no benefit to the store.
They're justifying it as a customer acquisition expense, but you have to pay for it in perpetuity. Not good.