I'm using Shopify Lite and have been happy with it over the past 2-years, as it's mainly a side business to our front business (being training and development) and I haven't had any need or want to upgrade to Shopify Basic plan.
Now, I'm wanting an option for my customers to add more products in their cart during checkout (I want to upsell them!). However, all the upsell/cross-sell apps are for Online Stores, and I have no need or want to add this feature due to additional admin to keep up yet another site (already run 2 others), and I do not want to bear the cost of upgrading, and an app cost.
Is there a way to just add this as custom code to the checkout? We mainly sell 1 product upfront (that's what our customers come to our website to purchase), and then I want to upsell them with another one of our products.