Items containing personal correspondence, which includes letters, brochures, catalogues, C4 and addressed to an individual, postcards, braille letters, and individual invoices/statements, are not goods and do not require a customs declaration.
Yes, regardless of service (tracked, signed, economy, standard) it's best to do this. The Universal Postal Union (UPU) has mandated that any international items containing goods must have a unique identifier (known as an S10 barcode).