| APA | Smart Define. (2011). 1st-class mail thesaurus. Retrieved September 20, 2026, from https://smartdefine.org/1st-class_mail/thesaurus/synonyms |
| Chicago | Smart Define. 2011. "1st-class mail thesaurus" https://smartdefine.org/1st-class_mail/thesaurus/synonyms (accessed September 20, 2026). |
| Harvard | Smart Define 2011, 1st-class mail thesaurus, Smart Define, viewed 20 September, 2026, <https://smartdefine.org/1st-class_mail/thesaurus/synonyms>. |
| MLA | Smart Define. "1st-class mail thesaurus" 26 April 2011. Web. 20 September 2026. <https://smartdefine.org/1st-class_mail/thesaurus/synonyms> |