These three terms are also very tricky. I hope this post clarifies.
Principal. Adjective: main. E.g.: Su principal logro hasta el momento, según coinciden en su mayor parte los analistas, es que no se han roto: Their main achievement to date, analysts tend to agree, is that they have not broken down.
Principio/s. Masculine noun, usually in plural: principle/s. E.g.: No confundan los valores ni menosprecien sus principios: Do not mix up your values or betray your principles.
Principio: Masculine noun, usually in singular. Beginning. E.g.: Desde el principio me involucré en muchos negocios: From the beginning I was involved in many businesses.
Príncipe: Male noun. Prince. The female form is princesa. Remember that in the Spanish Monarchy príncipe or princesa is just the heir to the Spanish throne, any other son or daughter of the Spanish king is an infante or infanta, pretty much like the term fils de France.
On another note, the English noun ‘principal’ in Spanish is usually translated as director/a de escuela. dc

Masculine noun. In Legal Spanish the acervo are all assets that belonged to a deceased person, and that must be presented before the judge who intervenes in the probate trial. It also called acervo hereditario or sucesorio. E.g.: Si usted no tiene cónyuge o hijos, sus familiares directos recibirán sus bienes y si no tiene familiares, el estado absorberá todo su acervo hereditario: If you have neither, your closest relatives will be the recipients, and if you have no relatives, your entire estate will be absorbed by the state. dc

Adjective. Both ológrafo and hológrafo are valid. This word is used almost exclusively in Legal Spanish and it refers to a document, especially a will, entirely in the handwriting of the person whose act it purports to be. E.g.: Una seria laguna, que habrá que colmar cuanto antes y sobre la que ya en el pasado se insistió, es la falta del testamento ológrafo de un número considerable de militares: One serious gap in our archives that must be remedied as soon as possible involves the hand-written last wills and testaments of many of our military personnel. dc

Masculine noun. In Business Spanish, flete is the cost of the shift of goods in a transport vehicle. E.g.: Los costos de flete y los gastos de manipulación relacionados con la venta se incluyen como un componente de las ventas netas: Included as a component of net sales are freight costs and handling charges related to the sales.
The verb is fletar and means to contract a vehicle to ship goods: to charter a ship? dc

Since many of my students mix up these words, I decided to write a post on them. I hope it clarifies. Please, take the quiz at the end.
Político/a/s. Masculine and feminine noun: Person active in party politics, or a person holding or seeking an elected seat in government. E.g.: En algunos casos es casi imposible procesar a un político corrupto que ha escapado de la jurisdicción de un país: In some cases the challenge of prosecuting a corrupt politician who has escaped from a country’s jurisdiction is next to impossible.
Política/s. Feminine noun, usually in plural: Deliberate system of principles to guide decisions and achieve rational outcomes. E.g.: Política pública de gestión de sustancias, productos y residuos definida que establezca los criterios rectores claves para el desarrollo sustentable: A defined public policy for chemicals, substance and waste management which determines the key governing principles for sustainable development.
Política. Feminine singular noun: Activities associated with the governance of a country or area, especially the debate between parties having power. E.g.: Ese evento me inspiró para seguir interesándome en la política: This inspired me to pursue my interest in politics.
Político/a/s. Adjective: Relating to the government or public affairs of a country. E.g.: En cualquier caso, es ilegal dotar de carácter legal a un texto de naturaleza política: In any case it is illegal to give legal character to a text of a political nature.
Policía. Feminine noun: Civil force of a state, responsible for the prevention and detection of crime and the maintenance of public order. E.g.: De momento la policía no ha encontrado el móvil del asesinato: Police have so far not identified any motive for the killing.
Policía. Masculine noun: Male police agent. Also used el/un agente de policía.
Mujer policía. Feminine noun: Female police agent. Also used la/una (agente de) policía.
Póliza: Feminine noun: document detailing the terms and conditions of a contract of insurance. E.g.: Contrata una póliza de seguro flotante por los objetos que estás trasladando: Take out a floating insurance policy for the objects you are relocating. dc