How to customize this shortcode which shows the number of views of a single post

Ah, that’s clever. You’re almost there:

function mywpp( $atts = NULL ) {
	extract( shortcode_atts( array(
		'id' => NULL,
		'range' => 'all'
	), $atts ) );

	$pID = NULL;

	if ( $id )
		$pID = $id;
	else
		$pID = get_the_ID();

	return '<div class="wppcounter">' . wpp_get_views($pID, $range) . '</div>';
}
add_shortcode('mwpp', 'mywpp');

I haven’t tested this, but it should work.

Let me know how it goes, alright?

Been too busy with work projects. At my job, we were short of personnel for a bit too much time and many projects got delayed – hence the load of work got too big and you know how that ends. I still have a lot to do (and will get even more work in the upcoming days) but decided to take this weekend to work a little bit on the plugin.

Anyways, do you really need to use a DIV element in there? How about this:

function mywpp( $atts = NULL ) {
	extract( shortcode_atts( array(
		'id' => NULL,
		'range' => 'all'
	), $atts ) );

	$pID = NULL;

	if ( $id )
		$pID = $id;
	else
		$pID = get_the_ID();

	return '<span class="wppcounter">' . wpp_get_views($pID, $range) . '</span>';
}
add_shortcode('mwpp', 'mywpp');

By the way, the default value that you have specified for the range, is all, right?

Yes, that’s correct. Also the post ID will default to the current post/page ID as well if you don’t pass it to the shortcode, so you could just use [mwpp].