Modify express checkout (Shop Pay, Paypal, etc) cart before checkout

[FlorSlabecki]

In my Shopify store above the add to cart button I have a checkbox for a user to add a customized letter for an extra $3, if the users checks the checkbox and goes to cart and then to checkout it works fine.
The problem is that I don't understand how to manage to make the customized letter appear when the user clicks one of the dynamic shop buttons/express checkout like Shop pay or paypal (these show below the Add to cart button on the product page.
How can I modify what's sent to these express checkout to include the customized letter?
As a note, the customized letter it's product on its own with a product id and variant id