u/Emergency_Apple_5468

i have a question,does somebody know how to add post purchase upsell for cod payments i have tried with couple of apps,but when i add the products it doesnt add it automaticly to the already made order but instead send you again to a new checkout.I have seen other stores which use cod payments and it automaticly adds the new product to the already made order.I will be very thankful if somebody helps.