Ad Serving Costs are any costs accrued from the actual running of an ad. These can include (for example) things like Ad server fees and 3rd party tracking fees.
This term does not usually include admin fees (such as paying an ad agency to book ad space). The rule of thumb is that if it costs you more for running more ads, then it counts as an Ad Serving Cost.
If it is a flat rate then it is an Ad Serving Fee.
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