ISSUE #2: Use wp_enqueue commands

[xecdev]

Your plugin is not correctly including JS and/or CSS. You should be using the built in functions for this:

When including JavaScript code you can use:
wp_register_script() and wp_enqueue_script() to add JavaScript code from a file.
wp_add_inline_script() to add inline JavaScript code to previous declared scripts.

When including CSS you can use:
wp_register_style() and wp_enqueue_style() to add CSS from a file.
wp_add_inline_style() to add inline CSS to previously declared CSS.

Note that as of WordPress 6.3, you can easily pass attributes like defer or async: https://make.wordpress.org/core/2023/07/14/registering-scripts-with-async-and-defer-attributes-in-wordpress-6-3/

Also, as of WordPress 5.7, you can pass other attributes by using this functions and filters: https://make.wordpress.org/core/2021/02/23/introducing-script-attributes-related-functions-in-wordpress-5-7/

If you're trying to enqueue on the admin pages you'll want to use the admin enqueues.

https://developer.wordpress.org/reference/hooks/admin_enqueue_scripts/
https://developer.wordpress.org/reference/hooks/admin_print_scripts/
https://developer.wordpress.org/reference/hooks/admin_print_styles/

Example(s) from your plugin:
includes/class-paybutton-admin.php:83 <style>.cls-1 { fill: none; }</style>