Styling?

Great! Two small suggestions.

On your first div in the output:
<div class="woocommerce wctc-coupon-outer" style="color:#680709;">
it might be an idea to add an ID there, that you don’t use yourself in your own css. Maybe:
<div id="wctc-coupon" class="woocommerce wctc-coupon-outer" style="color:#680709;">

That way, it doesn’t matter which order your css and the user’s css load (which occasionally, in some themes, can be unpredictable). Once the user writes #wctc-coupon it has higher specificity and will not be overridden.

—

You’re now outputting the plugin’s css to a file, which is great! However, that file is included on every page on the site, which seems a bit excessive. 🙂
Could you check to see if the page is “order-recieved” before inserting your css? Maybe something like:

if( is_wc_endpoint_url( 'order-received' ) ){
//Insert css
}