The left footer has the only gallery that will work on every shop page (or blog page, or many other kinds of page that we can't use the other method).
You just have to replace that gallery. The two screenshots from the previous post show you where the left footer is located. You should be able to simply edit it.
Granted, right now I've got a bug on my chrome browser which I can't properly edit that footer in any squarespace site. But I can do it on safari, for instance. Maybe you're having a similar issue.