I went through the past questions but didn't actually practice what i will say, so i messed up because i couldn't be concise, but if you prepare enough, it should be a breeze! go through the past questions on GLassdoor, it is very very very helpful. and also create a trial shopify account for yourself!
I'd say, "To be one of the paid workers", since it seems like they use most candidates to get free work, according to what I've read on here. Why would a company ask a candidate to answer real and current customer concerns? You would think they were well enough along to have complied the type that test for customer service ability and not NEED candidates to actually solve real customer issue without getting paid for it. They should be audited to see if that is what they are up to. It sounds suspicious with their high rejection rate with no cause. I hope they are report, en mass to the employment standards by candidates. Or that someone internally blows the whistle, if that is actually going on. Bottom line is, it has been the theme in all of these interviews so it MUST BE INVESTIGATED!