Costs are probably not that high anyway. For most logistic companies, volume determines capacity for trucks, not weight. Transporting a washing machine amongst other deliveries will not cost much more than other deliveries of that size (except for the additional fuel which is probably low). Carrying it in place can be done within minutes, not much longer than other deliveries.

That's probably also the reason why machines are cheap to buy online. If shipping was a constraint, buying at big stores would be cheaper.

Therefore, concrete can save some CO2 but probably not much cost. The cost of replacing faulty water tanks could be higher than the saved cost.