When you're theming a Drupal Commerce site, you shouldn't forget to style the emails your store sends. The primary use case is the checkout completion email that includes the customer's receipt, but there may be others depending on your feature set and customizations (e.g. dunning emails for recurring subscriptions).
Commerce Core provides a basic, table based template that you'll want to either replace or at least ensure is styled by your theme to reflect your branding. (Find it in the order module's templates directory, commerce-order-receipt.html.twig.) However, without adding the capacity to properly send HTML email from your site, it's going to look like a jumbled mess in your customer's inbox.
We've long recommended Swift Mailer for formatting and sending HTML emails from Drupal Commerce. Symfony announced the project's deprecation late last year, recommending folks switch to using the 3 year old Symfony Mailer instead after they brought it up to full feature parity with the Swift Mailer library.
Fortunately, there's a module for that!
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