Another thing a universal shopping cart would be good for would be unofficial partnerships. That is, I think we all know that actual legal partnerships can be problematic. Let's say that Jill and Jane share a similar audience. They want to work together but not have a formal partnership. With a universal shopping cart, they could do something like this:
Jill and Jane each have their own websites. Jill offers Jane's books on her site as an affiliate. Jane offers Jill's books on her site as an affiliate. If you go to either site, you could buy one of each author's books. Each gets their full payment on their own book and whoever's site the purchase was made on gets a commission on the other's book.
Or, let's say Jane doesn't want her own website. Jill could offer Jane's books on her site. Jane gets paid for each book sold and Jill, as an affiliate, gets a commission for each of Jane's books sold. Jill could also offer other authors' books on her site for an affiliate commission.
Or, you could have any kind of mix of methods. In any case, there is no formal partnership required because the money is being paid out by the merchant of record (Lemon Squeezy, Paddle, whoever). So, Jill never touches Jane's money and Jane never touches Jill's money. And there is no legal relationship (that I know of) required between Jill and Jane. The affiliate agreement would be between the website owner and the merchant of record, not between Jill and Jane.
So, this would allow a way of authors working together without having to form any legal partnerships because the only legal business relationships would be between the authors individually and the merchant of record.
And, in the case of messy incidents, clean up is easier because there are no partnerships. For example, if Jane runs off and murders her husband and feeds the meat to her kids, Jill probably isn't going to want to promote Jane's books anymore. With a legal partnership between the two, that may require lawyers and stuff. Hopefully, there would be a dissolution clause to cover stuff like that in a partnership agreement but that's assuming they had one. But, in our example, since there is no partnership, Jill can take down Jane's books at any time after hearing about the cannibal murder. And that makes things relatively easy and less messy than a legal entanglement.
Of course, that's not to say that Jane might not sue Jill anyway, but Jane would probably have less of a case when there was no legal partnership and Jill was listing her books as an affiliate. Plus, you know, the merchant of record might pull the books first, which takes any heat off of Jill entirely.
Disclaimer: I am not a lawyer and also haven't stayed at a Holiday Inn recently.