For an online retailer getting users to complete the order is the most critical step. That we can all agree on.
What’s harder to agree on is the phrasing: is it checkout (one word) of check out (two words).
From a grammar perspective Writing Explained has a solid explanation:
Check out is a verb phrase that means to sign for something or to observe something. Checkout means a place to buy things or the process of leaving a hotel as a noun, and it describes qualities of these things as an adjective.Writing Explained
Based on definition above I’m leaning towards declaring checkout is the way ecommerce retailers need to use it on their sites.
Unfortunately, it doesn’t help when the two largest retailers on this planet don’t see eye to eye: