Most mail programs have limited understanding of styling, if any. I'd go with attaching a PDF and providing a link to your web site where the receipt can be shown as HTML.

In reply to Re: PDF vs. HTML for invoice generation by ikegami
in thread PDF vs. HTML for invoice generation by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
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