Steve Grunwell

<a href="https://codex.wordpress.org/Excerpt" rel="nofollow ugc">WordPress post excerpts</a> can be a great way to hand-craft the summary of your content. Unfortunately, writing post excerpts isn’t as nice of an experience as what you find elsewhere in WordPress. Want to include links in your excerpts, or bold some text? Hopefully you know some HTML!
Advanced Post Excerpts is designed to change that, by giving your editors an easy, intuitive interface for writing great post excerpts.
For complete details and/or to contribute to ongoing development, please <a href="https://github.com/stevegrunwell/advanced-post-excerpt" rel="nofollow ugc">visit this project on GitHub</a>.

Advanced Post Excerpt

Replace the default Post Excerpt meta box with a superior editing experience.

slee

This plug-in allows you to add code such as a PayPal donate button to end of a post or page. 
You can select certain post categories or certain pages to show the content or have it shown on all content.
<h3>Support</h3>
Support is provided at http://www.thefunkhouse.co.uk/wordpress-plugins/end-content

End Content

Allows you to add content to the end of pages, posts or both.

Advanced Post Excerpt vs End Content