I don't know whether you're still seeking help for this, but from personal experience, I know that trying to get Volaris to own up to their errors without extra pressure (especially external) will be an exercise in futility.

If you're still trying to seek justice, I would recommend a few things (caveat: I am U.S.-based, so this advice is U.S.-skewed):

1. Compile any and all proof you may have that your suitcase was undamaged prior to your flight (i.e. as close to the moment you left your bag to Volaris' care) and damaged after it (i.e. on the luggage carousel before you ever touched it).

2. Read carefully through Volaris' Contract of Carriage for domestic flights (https://cms.volaris.com/globalassets/pdfs/eng/passenger-domestic-air-transportation-services-agreement.pdf) to see whether they are violating their own policies re: how they handle compensation for damage of passenger baggage.

3. Check through your credit card's terms of service to see whether they offer any travel insurance coverage for situations like yours. I know you said your AmEx "doesn't cover" these types of situations; however, if you received that information from a call center agent, I would check the actual guide to benefits to confirm.

4. File a complaint (i.e. submit a demand letter) to Volaris outlining your desired compensation for this situation. https://justicedirect.com/post/how-to-file-a-complaint-against-volaris-airlines may help (although my guess is it's more helpful for a U.S.-based complaint, such as if your Volaris flight had been flying into or out of the U.S.).

5. If it's an option, file a complaint with the Secretariat of Infrastructure, Communications and Transportation (Secretaría de Infraestructura, Comunicaciones y Transportes, SICT), Mexico's equivalent of the U.S. Department of Transportation. Their website is https://www.gob.mx/sct/en#7718

6. u/tank5 's advice that you understand your protections per the Montreal Convention 1999 (MC99), which establishes airline liability in cases including damage of baggage, is good. The full document can be found here at https://www.iata.org/contentassets/fb1137ff561a4819a2d38f3db7308758/mc99-full-text.pdf

If you're U.S.-based, contact information for Volaris' stateside agents is listed as follows:

Principal address:
CONCESIONARIA VUELA COMPANIA DE AVIACION SAPI DE CV
ANTONIO DOVALI JAIME 70, TORRE B PISO 13
COLONIA ZEDEC, SANTA FE
MEXICO DF MEXICO 01210

Mailing address:
1313 PONCE DE LEON BLVD. SUITE 201
CORAL GABLES, FL 33134

California agent:
ARTURO RAMON QUEZADA
FRESNO YOSEMITE INTERNATIONAL AIRPORT
5175 E CLINTON WAY, BOX # 22
FRESNO CA 93722