Question

SELECT 
    SUM(
        CASE 
           WHEN cumulative = 1 
           THEN percent 
           ELSE 0 
        END) 
FROM phppos_items_taxes;

Given the above statement does this do the following:

mysql> select * FROM phppos_items_taxes;
+---------+-----------+---------+------------+
| item_id | name      | percent | cumulative |
+---------+-----------+---------+------------+
|       1 | Tax 1     |    8.00 |          0 |
|       1 | Tax 2     |   10.00 |          1 |
|       3 | Sales Tax |    8.00 |          0 |
|       4 | Tax 1     |   20.00 |          0 |
|       4 | Tax 2     |   20.00 |          0 |
+---------+-----------+---------+------------+

Does this SUM up percent for each row that cumulative = 1. If cumulative != 1 then 0 is summed.

Answer 1

Yes it does!
A shorter and cleaner query (IMHO) would be to use IF statement:

SELECT SUM(IF(cumulative = 1, `percent`, 0)) FROM phppos_items_taxes;