This is the hardest question I've answered all day.
One way to find a sub-total is by *simply* adding everything together, and a example would be below, buy yeah, you just add everything together.
For example:
Cookies (item # 594993) $2.98
Cake (item # 4354533) $4.50
Vans low tops (item # 594939) $60.00
Vans socks (item # 598993) $15.00
And just by adding all this, that how you would find the subtotal. Hope this helps! ;)