Hi,
Not sure if it’s a problem with the current version of WP (a change in the keyword?), but with 4.5 it looks like a typo:
echo get_the_post_thumbnail( $product[0]->ID, 'post_thumbnail' );
The above shows resized original images, not the thumbnails.
After changing the _ to – the thumbnails have correct sizes:
echo get_the_post_thumbnail( $product[0]->ID, 'post-thumbnail' );
Cheers,
Tom