i don't see how this is a problem that would need Ars as a mediator.
Craftsman: i want to order product A
Merchant: sure thing
a week later
Merchant: here's product B
Craftsman: i didn't order product B i ordered product A
Merchant: whatever i brought a product so gimme moneh!
it should be very clear that you can't force someone to buy a product they didn't want or order, the merchant is 100% in the wrong here.