Didn't I warned that this mod comes without any warranty
It worked for me but failed in your specific conditions.
Here is an alternative mod:
In the same function find the lines:
#
# Calculate total
#
$total+=$shipping_cost+$tax_cost+$tax_gst+$tax_pst +$giftcerts_cost;
And replace it with:
#
# Calculate total
#
if ($customer_info[membership] == 'Wholesale') $tax_cost = 0;
$total+=$shipping_cost+$tax_cost+$tax_gst+$tax_pst +$giftcerts_cost;
The added line unconditionaly set US taxes to zero if membership is Wholesale.