If merchant don't want to process the jet order, Then merchant can reject the order from sales->Failed Sales Order panel.
Here merchant have to enter the Merchant Order ID and Order Item ID for particular order. And the order will be rejected on jet.
Please once go through the below shared Doc(how-to-sell-on-jet-com)
https://shopify.cedcommerce.com/jet/how-to-sell-on-jet-com#sec6-5