Мы считаем, что первое предложение достаточно эффективно, это привлечет постоянных покупателей. Но, возможно, стоит изменить количество бонусных литров в сторону увеличения, поскольку один литр - это очень мало.
We believe that the first sentence sufficiently, it will attract loyal customers.But, you might want to change the amount of bonus litres upwards because one litre is very little.
We believe that the first sentence effectively, it will attract loyal customers. However, you may want to change the number of bonus liters upwards as one liter - it is very small.
we believe that the first sentence is effective to attract the customers.but, maybe we should change the amount of bonus of upward, since one liter is very little.