Tax issue

Hi, I am using your plugin and it working fine. However, it is calculated in different ways for different shipping methods. But, local pickup are working fine. I have set up a standard tax in percentage . e.g my product price is 16.75 and tax rate is 6.875. So, it should calculate 1.15.. And it is giving result fine in local pickup. But, tax is showing $4.40 when I select Priority Mail Express:?$47.25. And showing $1.88 when I select Priority Mail:?$10.59. But I want to calculate standard rate for all of shipping.