Generated code for css classes might be out of place.
I've noticed a couple of times that elements where either showing twice (qty for example) or they where not showing up at all on mobile devices.
I figured out that it was an error in how classes where sometimes placed in the id instead of the class for that element. I fixed them thinking I had somehow moved them but now I see them again on the success page which I just started messing with.
Here's an example.
<p class="eC_FormItem eC_ItemQuantity" id="@@CartName@@_JSON_Quantity max_tablet">1</p>
<p class="eC_FormItem eC_ItemQuantity" id="@@CartName@@_JSON_Quantity desktop_only">1</p>
It looks like the file in eCart6 might be writing the class "max_tablet" and "desktop_only" to the id instead of the class which is causing the css to hide/show to fail. An easy fix but I wanted to bring this to your attention in case others have this issue.