Mexico does have an "IVA" (Impuesto al Valor Agregado - Value Added Tax) at the rate of 15% in most places (not imposed on certain purchases such as medications, some foodstuffs, etc.)

Most lodging etc. quote it included in fares or costs, a very few less scrupulous may not. It sounds like Expedia is "winging" it and not making much sense - the air package purchased in the US or outside of Mexico should certainly not include any IVA.