From this perspective there is no difference because the money the rich will spend in buying the goods ones to last for two months is exactly the amount the poor might spend getting it everyday for two months, so actually the one time purchase is what makes a little different about it, although the consuming method might differ a lot from that of the poor because a poor person takes there food items slowly knowing that it cost them a lot to get but the rich doesn't regard that and they are ready to use it even when is not necessarily for them, so for economical part of it the poor does better.
There is a big difference between a person who buys goods in large quantity and a person who buys in small numbers, for example a person can spend $50 in getting a carton of biscuits spends less than a person who buys the biscuits one daily. The consuming method of the poor is faster than that of the rich you will agree to this because the poor eats more than the rich.
If that's the point definitely the poor might be spending more than the rich because there system of buying is already positioning them to spend too much on a something they isn't supposed to spend that, but however for the fact that the rich could have even gotten it twice before the poor will have their first unit as the rich has makes it higher for the rich to spend, for instance before the poor will purchase up to a carton with there system of doing it the rich could have even exhausted more than three cartons already, so when you calculate it this way if the carton is supposed to serve a month the rich are the ones spending too much even with there wholesale buying.