So you can add class like this:
<p class="affiliate-notice"><em>your-text</em></p>
Then edit your CSS to:
.entry-content p, .entry-content ul, .entry-content ol, .entry-content h1, .entry-content h2, .entry-content h3, .entry-content h4, .entry-content h5, .entry-content > div, .entry-content blockquote, .comments-area, .affiliate-notice {
max-width: 700px;
text-align: left;
word-wrap: break-word;
margin-left: auto;
margin-right: auto;
clear: both;
}