A client has complained that their website is slow, so I'm trying to implement some improvements. The first thing I was going to do was modify their htaccess file but, being that they are using a Shopify system, it doesn't seem possible to do such a thing.
How can I access and modify the .htaccess file for a Shopify website? Or if I can not access .htaccess, how can I enable GZIP compression, Expire / Cache-control, etc.. How can I setup all the things that would normally be setup in an .htaccess file?
You cannot DevOps a slow Shopify site.
Shopify is managed by a DevOps team far more skilled than Apache tweakers used to fiddling with that venerable ancient beastess and .htaccess files.
You could tell you client their website is slow most likely (read 90% chance or better) due to poor theme coding. Check their Javascript external service loads. Check their CSS. Their images for optimization. Almost all slow Shopify sites are due to sloppy, incompetent coding of those front-end concerns. Shopify uses a CDN for assets that is global, meaning those in places poorly served by the Internet will remain slow, but for everyone else, things should be fast.
And they know all about compression and caching too. You are likely barking up the wrong tree.