There are a few ways you can have a "Buy now" button take a customer straight to checkout, but I am not aware of any that allow you to auto-fill payment details. Maybe there are apps that can do so?
Have a look at dynamic checkout buttons to see if those are good enough for your use case.
Hope this helps!
You can get part of the way there if you allow people to create accounts. You can find this in settings -> checkout. If you're using Shopify Payments, and you let people save their info, it will pre-populate if they enter the email or phone number they signed up with previously.
You can read more about it here.
Is that helpful?
ShopClerk Order Tracking for Shopify
|2 hours ago|
|10 hours ago|