It is common practice now to add an upsell app post purchase. Shopify supports updating orders after the order is created - with apps like One Click Upsell from Zipify and Carthook.
This was a popular requested feature on the previous Shipstation communit board before that community board was depepricated. If shipstation supported order edits, support for shopify order holds, or had the option for a delay on orders these apps and the upsell add to cart would work as expected.
Would love to see this supported with Shipstation soon.