Infopath calculation error with decimal points

[Joe]

I am creating a purchase request form in InfoPath and whenever I put values
in the form I am having the tax / total, etc. calculating. However when it
calculates the result has more than 4 decimal places and returns an error.
I've indicated that I only want 2 decimal places, and that is all it shows,
but it still gives an error.

per-item tax calc: @pre_tax_price * (1 + (@tax / 100))
Tax calculation: sum(@price) - sum(@pre_tax_price)
Grand Total Calc: sum(@price) + @ship_handling

The form has these fields:
Qty [ ]
Unit Price [ ]
Tax (%) [ ]
Pre-Tax Price [ ] (auto calculated)
Total with Tax [ ] (auto calculated)

then below I have these fields
Subtotal - I get the error here first
Tax
Shipping
TOTAL - then I get the error here

 

[S.Y.M. Wong-A-Ton]

What is the error message you got?