IMPORTANT!
This extension requires VQMod Installed!
Upload and go — utilizes vQmod so no core file modifications are necessary.

Subtotal doesn't include voucher amount, so the shipping is based only on subtotal. With this VQMOD, subtotal include voucher amount.
For example: you offer free shipping on 150$. If you have an order with the total 180$, and customer apply voucher with 50$ amount, total will be 130$, but shipping remain free. With this module, shipping will not be free anymore, if the total order (sub-total+voucher) is less than 150$.
Tested on 1.5.6. Feel free to leave your feedback.