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If you want to add the sharethis buttons to post-pages only, you will have to add the sharethis code manually to your templates since this isn't possible with the sharethis plugin.
You can get the code (by selecting website platform is step 1 from here:http://sharethis.com/publishers/get-sharing-tools)
Please note that you will have to modify the span tags as explained below.
Sharethis script and span tags go on the page where you want the buttons to appear and they also determine what kind of button appears as well as what URL and title it shares. You will have to put these in your Single Post template (single.php) in order to add the buttons only to the post-pages.
Depending upon the position of buttons on a page, you will have to decide the position of the code in th file single.php
You can add:
Sharethis script tags below the line:
span tags below the line of code:
<?php get_template_part( 'content', 'single' ); ?>
Here is the basic span for just the ShareThis button:
<span class="st_sharethis" st_title='<?php the_title(); ?>' st_url='<?php the_permalink(); ?>' displayText="ShareThis"></span>
You can change the class of the span to determine the button that appears. For example class="st_twitter" will make a twitter icon and class="st_email_vcount" will make a email vertical counter button.
You can read more about the different types of buttons here: http://help.sharethis.com/customization/customization-overview
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