The checkout button on Gift Reggie auto-fills the registrant's address. However, to make the checkout button on the cart page auto-fill the address, please follow the below instructions.
1. Go to the online store ->Themes -> Customize your current theme
2. Go to App Embeds -> expand Registry and Wishlist
3. Scroll down, find "Checkout Button Selector (Advanced)" settings and add your checkout button class name here.