Want to show some items pre-selected on your checkout page?
It could be to give a visual cue to users about the popular choice in your store.
Take a look at how the product list appears on the checkout page:

Step 1: Go to the 'Products' section and choose the option to let buyers select multiple items
Choose from one of these options:
"Allow the buyer to select any of the above product(s) (e.g. when selling multiple products)"
OR
“Restrict buyer to select only one of the above products (e.g. when selling similar products with different pricing plans or quantity)”

Once done, click on the 'Save Changes' button to save the changes.
Step 2: Go to the 'Fields' tab and click on the 'Products' field

You'll see the field editor popup open.
Step 3: Click on the "General" tab and select the product(s) which you want to add to the cart by default
Check the box against your default options.

Next, hit the 'Update' button.
Step 4: Click on the 'Save changes' button to save the changes you made

Now, the selected products will be pre-selected in the cart on the checkout page.