Sort column by invoice number doesn’t count correctly

Sorting of unformatted invoice numbers is problematic because there can be many different formats. Most shops actually apply a padding (000010, 000123 etc.) and/or prefix to the number and we can’t sort an invoice ‘number’ like XYZ000123-WEB as an actual number (integer) because it would give unexpected results. For that reason we have chosen to do a flat alfanumeric sorting. If you want to override this and apply numeric sorting, you can use a filter:

add_filter( 'wpo_wcpdf_invoice_number_column_orderby', function(){ return 'meta_value_num'; } );

If you haven’t worked with code snippets (actions/filters) or functions.php before, read this guide: How to use filters