No Add to Cart, only buy now then go to checkout

No add to cart, only buy now then go straight to checkout.

cannot add other item, one product one time checkout.

is it possible?

Sure. You will need to change the button action from checkout.add to checkout.checkout in the buttons/checkout.tpl template and pass the appropriate product_id. You will have to read the controllers/customer/checkout.php controller for details.

thank you for your help!