@jmellin The best solution would be to tell ZenCache to exclude those pages from the cache. Normally the eCommerce plugin will do this for you (plugins like WooCommerce already tell ZenCache and other WordPress caching plugins which pages should not be cached), but if you’re using a plugin that does not properly support WordPress caching plugins, then you’ll need to tell ZenCache to exclude the dynamic pages (such as the checkout page and shopping cart page).
To exclude a page in ZenCache you need to specify a URI Exclusion Pattern (ZenCache → Plugin Options → URI Exclusion Patterns). That’s currently a Pro-only feature, so if you’re using the free version of ZenCache you won’t have that option. However, we’re making that feature available in the free version as of the next plugin update, which should happen within the next week.