Ecommerce Single Product Next Product Link

next_post function is deprecated.

You should try this instead: next_post_link and previous_post_link

The basic way to use it is like so:
This will link to the next post or prev post.

<?php 
    next_post_link( '%link', '%title' );
    previous_post_link( '%link', '%title' ); 
?>

If you would like to link to the next post in the same category, add true after ‘%title’. This will work for the default posts and category.

<?php 
    next_post_link( '%link', '%title', true ); 
    previous_post_link( '%link', '%title', true ); 
?>

If you are using a CUSTOM POST TYPE with a CUSTOM TAXONOMY (like a product post), then you have to tell the function which taxonomy to look for the next post. eg.

<?php 
    next_post_link( '%link', '%title', true, '', 'custom_taxonomy' ); 
    previous_post_link( '%link', '%title', true, '', 'custom_taxonomy' ); 
?>

All the above examples have been tested and working.