First, all Cashu devs I'm connected with say their code/service is experimental and to only use small amounts that you don't mind losing to unknown defects.
So Shopstr is based on a (currently) non-working system.
Then, it seems, this Cashu requirement isn't communicated to the user at all, when it should be stated before they list an item.
In fact, everything that is required of the user to complete a purchase should be communicated before they make a listing.
If I made a sale, and found out that I received Cashu, I would be totally pissed off. My experience with Cashu is limited, but I couldn't convert to sats. Screw that.