Coupon discount calculation and Tax

Hi,

Can someone please help to resolve below two issues:
(1) when coupon used in the cart, some how plugin force the cart calculation to exclude coupon value. Once clicked on anyother shipping method and come back to Transdirect method then it does consider it.
(2) plugin force the cart to calculate tax on coupon value. There are multiple shipping calculation methods but unfortunately, only Transdirect plugin miscalculate the GST in cart.

For example; item price ex GST is $100, $20 coupon, 10% tax value, $10 shipping Incl. tax,
Transdirect calc: In first attempt 100+10 GST on product + 10 shipping incl GST = $120. Here cart ignored coupon discount only when Transdirect plug in is selected. When switched to other method and come back to Transdirect it does consider the coupon but calculate GST (10%) on full amount. So calculation looks like below:

100 product – 20 coupon +10 GST on product (it should be $8) + 10 shipping incl GST = $110.

Correct calculation should be: 100 product – 20 coupon = 80 discounted base value +8 GST on product + 10 shipping incl GST = $98.