Advertise Discount Codes on Checkout or on Product Page

[cgcl2]

I'm looking for a way to advertise my discount codes at checkout when someone is ordering a specific item featured in the discount or on the product page when someone is looking at it. I am not enthusiastic about or really willing to pay an additional fee for a app specific too this. I am using the Craft Theme. 

I make cages for tractors and the door kits do not come in the main kit. I want to advertise the discount I have already created to entice people to purchase the door kit as a combo with it at a hefty discount but I need a way to advertise when they are on the product page or when they have the item in their cart. The best way I see I can do this right now is just adding the discount code in the product description of each item and I am willing to do that before purchasing a subscription for an app. 

[cgcl2]

I do not have any experience with html code, but I assume I would edit "Main Cage Kit" to be, for example, "Kubota M4/M5 Cage Kit". If I wanted this to be a more broad term for all of my cage kits that include door kits how would I do this? 

I plan on setting up one discount code to include all of our cages to be combo'd and discount the door kits for each individual. 

[cgcl2]

I went ahead and created a collection containing all the items I want included in the discount. How would I go about specifying a specific collection that is created on my storefront? Once that is done, what is the process for placing the code and being able to edit where the popup ends up on the checkout page? I think it may be easier to adjust this as you are watching live on the storefront. Could I contact you on discord or another platform?