Express Checkout for WooCommerce

Hello Team,
We are currently working on developing our client’s website using custom WooCommerce WordPress development, as described in the documentation found at https://woocommerce.com/document/template-structure/.
Once the development is complete, our plan is to incorporate an express checkout feature into the custom-developed website. However, we have encountered some issues during this process.
Currently, our website includes a donation feature where users can select a donation amount on the cart page. The chosen donation amount is automatically added to the cart total.
In order to enhance our website’s functionality, we purchased the plugin i.e.express checkout.
Unfortunately, upon installing the plugin, it completely altered the cart layout, reverting it to the default appearance. Moreover, the checkout page also underwent significant changes, resulting in the loss of existing features on both the cart and checkout pages.
We are seeking guidance to prevent such conflicts between the custom development and the plugin. It would be greatly appreciated if you could provide us with specific guidelines or steps to achieve the desired functionality of an “express checkout” while maintaining our current features on the cart page.
For your reference, we would like to share the following URLs:
Websie URL : http://wolper.co/
Cart Page URL: http://wolper.co/cart/
Checkout Page: http://wolper.co/checkout/