Integrating payment processing functionality into a third-party application via software development kit (SDK) instructions is described. In an example, a payment card reader can include reader instructions executable on the payment card reader to configure the payment card reader to read payment card data from a payment card and transmit the payment card data to a mobile device for processing by SDK instructions on the mobile device. The SDK instructions can be executable on the mobile device to configure the mobile device to process the payment card data from the payment card reader. The SDK instructions, which can be provided by a payment processor, can be configured to be integrated into a third-party payment application on the mobile device, wherein the third-party payment application is provided by an entity distinct from the payment processor.