Adding a suggested product to checkout page

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My site has only two products now. It will never go above 10 products. 

I'd like to have a single add-on product or order bump on the checkout page BEFORE they buy. Just like I've been doing in Sendowl. I want to stop paying Sendowl $25/mth so I'm looking for a workaround.

From my landing pages, I send people directly to the checkout link. That's why I'm looking for this option.

Can I simply add code to allow this to happen? Is there an app I'm unaware of?