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https://click2mail.com/reply-postcard-4-25-x-6-426

https://www.postcardmania.com/blog/mailing-101-business-reply-cards/

https://www.theaddressers.com/double-reply-postcards

https://click2mail.com/by-product/postcards

https://www.reddit.com/r/postcrossing/comments/1d3ovbd/is_it_ok_to_send_the_postcards_in_an_envelope/

https://pe.usps.com/businessmail101?ViewName=Cards

https://www.vistaprint.com/marketing-materials/mailing-services-postcards?srsltid=AfmBOop6_eHlMjyTe69zQe8JHkrSbHH2ta21NMc3mvp5VJAqQnrVz0LB

https://community.postcrossing.com/t/what-is-the-biggest-thing-you-attached-to-a-postcard-and-got-away-with/166454

https://www.reddit.com/r/USPS/comments/163x1nl/can_you_send_thick_postcards_in_the_mail_ones/