Methods and systems for integrating same-day delivery services with an online retailer platform are disclosed. Graphical user interfaces are displayed on a retailer website for browsing and selecting items for purchase. Delivery options are provided including those for same-day delivery. A delivery service can provide available schedule delivery timeframes to be displayed on a GUI of the retailer website. A customer selects a delivery window for delivery of items selected for same-day delivery. The delivery windows are communicated through an integration API. Features presented to the customer during a shopping experience from item selection to checkout can be seamlessly presented to the user from both the retailer and the delivery service. Payment can be managed by the retailer rather than passed through the delivery service.