They have the option to accept it. Technically, if you figure out the merchant's public key, you could "force" them into being a part of the "escrow" [1], but unless they agreed to it (and agreed to the arbitrator being used) they won't consider the product paid until the payment is under their control.

[1] I'm using quotes because the way it works with multi-signature isn't an escrow, but escrow is commonly used when talking about something like this. There's some explanation about how multi-signature transactions works and why its not an escrow in Bitrated's FAQ, under "How does Bitcoin arbitration work?" and "Is this an escrow?" (https://www.bitrated.com/faq.html)