Can someone tell me how to fix this calculation issue:

I ill show it on example, numbers are not adding up correctly, probably due to decimals, or something else. please help!

In example customer bought 2 of the same product and this is what i see in invoice:

Product 2
Tax 0%
Price (exc) 18.17
Price (inc) 18.17
Total (exc) 36.33
Total (inc) 36.34
.................................................................
Sub-Total: $36.33
Free Shipping Options (Free Shipping): $0.00
Sales Tax: $0.00
Total: $36.33


please help fixing it! 18.17 +18.17 should be 36.34

zen-cart 1.3.9h
currency US dollars.
Decimal places 2
Value 1.00000000