I'm currently working on version 2 of our Shopify app where we'll be adding custom Slack slash commands. When adding slash commands to a custom app, Slack requires the receiving server to have a trusted (not self-signed) SSL certificate.
Normally, what we do is simply create a reverse SSH tunnel to our stage server which is publicly accessible to receive webhooks, for example:
ssh firstname.lastname@example.org -R 8000:localhost:80
The above example will basically forward any traffic ...