coupon cause tax wrong?

[applese]

I am testing my zencart. The problem shows up

I add coupon then tax is wrong. after remove coupon then tax correct.
anybody can tell me where is wrong? ThankS!!

[applese]

if only one item need to check out, tax is correct after coupon is subtracted.
but if you try more than one item to check out, tax must be wrong with coupon.

my zencart is 1.3.7.1, php is 5

[applese]

but shipping fee is wrong