Enable Gift Details Before Payment
For any giving page, you can customize the donor flow from start to finish. One of the components that can be changed is the location of the 'Gift Details' section in the donation process. By default, gift details are at the end of the donation flow (post-payment information). To prompt the donor for gift details before payment information, use the checkbox in the Page Details tab to enable gift details prior to the payment information.
This is how the donation flow will appear to a donor, with the Gift Details enabled to prompt for gift details before payment information.