How to add custom "pay now" button and after submit, redirect to the checkout/orderId page
Use a rule. Example:

{ "rules_forward_user_directly_to_checkout" : {
    "LABEL" : "Forward user directly to checkout",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "TAGS" : [],
    "REQUIRES" : [ "commerce_order", "rules", "commerce_cart" ],
    "ON" : { "commerce_cart_product_add" : [] },
    "IF" : [],
    "DO" : [ { "redirect" : { "url" : "checkout" } } ]
  }
}


Note that you don't need to redirect to checkout/%order_id, you just need to redirect to checkout and Commerce will take care of the rest.