Well yeah, withdraw supplies does exactly what you say. If it fails to withdraw, it will buy them at mana shop or just ignore their absence (rings/amulets). It's mainly for you to buy health potions at market and withdraw them, so you waste less.
If it's not withdrawing money then you don't need to refill them at that time. If it says withdraw 0 for example, then there's something wrong with the way you set the amount of supplies.