The invention relates to a computer implemented system and method for payment flow integration using a pre-built user interface. The method comprises receiving, at a payment service provider, an API call from a partner server that specifies customized cascading style sheets (CSS) data for the partner. The customized CSS data allows the partner to customize the look and feel of a payment window that is embedded in the partner's webpage. The payment service provider server embeds the custom CSS data into an iFrame and transmits the iFrame URL to the partner server. The partner server embeds the iFrame in the partner's webpage. The method may also comprise receiving the user's payment information at the service provider's server, securely storing that information, and returning a payment token to the partner for enhanced security of the payment transaction.