We propose that consumers have mental budgets for grocery trips that are typically composed of both an itemized portion and in‐store slack. We conceptualize the itemized portion as the amount that the consumer has allocated to spend on items planned to the brand or product level and the in‐store slack as the portion of the mental budget that is not assigned to be spent on any particular product but remains available for in‐store decisions. Using a secondary data set and a field study, we find incidence of in‐store slack. Moreover, we find support for our framework predicting that the relationship between in‐store slack and budget deviation (the amount by which actual spending deviates from the mental trip budget) depends on factors related to desire and willpower.