At no point. I don't even consider it a matter of "responsibility". You can only take responsibility when things are clearly laid out. But what about the even more important responsibility to CLEARLY lay things out that should fall into businesses?
What I do expect and find important, and should be legally binding, is that no business should be allowed to hide the actual impact of purchases behind legalese and long-winded "financial product" descriptions, and then put the blame on the customer for not "reading through it".
At the very least, they should be explicitly, and in common, everyday language, give something like: "The average buyer that used this financing option ended up paying X% above the base price".