How can I understand the differences between two ISTA test procedures such as ISTA 6-Amazon.com-SIOC and ISTA 6-Sam's Club for my specific packaged-product?

Unfortunately, ISTA doesn't currently have a test comparison tool so the task will be a manual one. There are a couple ways to tackle this task,

Overviews - Compare test overviews from the ista.org webpage. All ISTA Overviews Test Procedures - International Safe Transit Association (ista.org); Sam's Club linkhttps://www.ista.org/docs/6SAMSoverview.pdf and 6-Amazon.com-SIOC link https://www.ista.org/docs/6AmazoncomSIOCOverview18-18.pdf
PackSight - ISTA's online test planning and reporting platform would allow you to generate a packaged-product specific test plan for each ISTA test protocol such as Sam's Club and 6-Amazon.com-SIOC for direct comparison. Access to PackSight is through an ISTA certified test lab or purchase of a user seat license. More information at the following link... PackSight - International Safe Transit Association (ista.org)
ISTA Certified Lab - An ISTA certified lab would be able to assist in review of the information and or creation of a packaged-product specific test plan comparison. You can find a lab near you using ISTA's Find A Lab search tool at the following link ... International Safe Transit Association (ista.org) ISTA certified labs are independent business so please contact them directly to discuss options and any associated costs.