Typically overhead is only charged on a portion of expenses. In our case, anything over $5k or that is part of a "constructed equipment" over $5k (these two categories are the large majorities of expenses in our lab, as most things we buy are components of detectors we build) are overhead free. Supplies/laptops/travel/tools/business meals/inexpensive equipment do incur overhead, but the effective overhead rate is much less than the nominal one.