@support
This should do it but maybe some improvements on css....
In style.css search for #item .sticky-button
Add:
#item .sticky-button.center {background:red; position: fixed;bottom: 58px; margin-left: 40%; right:40%;}
In item.php find:
<?php if(getBoolPreference('item_contact_form_disabled') != 1) { ?>
<a href="<?php echo eps_item_fancy_url('contact'); ?>" class="open-form contact btn btn-secondary sticky-button isMobile" data-type="contact">
<i class="fas fa-envelope-open"></i>
<span><?php _e('Send message', 'epsilon'); ?></span>
</a>
<?php } ?>
Under that line add:
<a class="sticky-button btn center isMobile" title="sometxt" href="https://your-link.com" onclick="return true;">
<span>some txt</span>
</a>