Order Total Modules making incorrect calculations! Help!

[shellbell]

Hi there! I've done many searches for this info and I just can't find anything so I thought I'd help and see if there's anyone out there who can help me!

Ok so store info: Running Zen Cart 1.3.8

Here's a description of the error....

MOST items in the order totals list on the bottom of an invoice are calculating incorrectly.

eg. ... I have 10% GST Tax and that is set up generally working ok. But even it is calculating incorrectly....

Here's an example of an order and the list of stuff at the bottom of the invoice

Sub-Total (inc GST): $140.13 (Tax is set to "Display")
5% Newsletter Discount: -$7.03
10% Discount Coupon: -$14.50
Flat Rate Shipping: $6.00
GST Total: $11.33
Total: $125.16

Now I think it should read...

Sub-Total (inc GST): $140.13 (Tax is set to "Display")
5% Newsletter Discount: -$7.01 (Where's the extra 2c come from?)
10% Discount Coupon: -$14.02 (The extra 48c?)
Flat Rate Shipping: $6.00 (GST is added so I chose to put in 5.4545 in the set up - more about that below)
GST Total: $11.37 (The missing 4c?)
Total: $125.10 (Where's the extra 6c come from?!)

Settings for each of the OT modules is as follows...

Sub-Total Module Settings

Sort Order = 1

Newsletter Discount Module Settings

Sort Order = 2
Include Shipping = false
Include Tax = true
Re-calculate Tax = Standard
Newsletter Type = zencart
Discount Amount = 4.5454% (I have discovered that including tax and having the discount amount as the non-tax amount gets the Discount shown on the invoice CLOSER to the actual 5% required)

Discount Coupon (ot_coupon) Module

Sort Order = 3
Include Shipping = false
Include Tax = true
Re-calculate Tax = None
Tax Class = GST
Shipping Cost 5.4545
Tax Class = GST
Tax Basis = Shipping
Shipping Zone = Australia


ot_shipping Module (Flat rate for this example)

Sort Order = 4

(This information is out of the flat rate shipping module FYI)

Shipping Cost = $5.4545
Tax Class = GST
Tax Basis = Shipping
Shipping Zone = Australia
(Again, I want the GST to include the shipping costs thus have put the cost in at the non-taxed amount and Zen Cart adds the GST and customers get charged the $6)

ot_tax Module (Flat rate for this example)

Sort Order = 5

ot_total Module

Sort Order = 6

Ok so I think that’s everything that should help to perhaps show up what’s going on. As far as I know, I have not edited ANY of the files related to any of the above Order Total Modules. So what’s going on?! Can anyone tell me what’s going on? I just don’t understand! I’ve played around with different set up options of all the options above and I just can’t get the numbers to come out right!

Any help would be GREATLY appreciated.

Thanks in advance.

[shellbell]

Even in the GST mod. What I'd REALLY like is that the GST mode just calculates the GST from the Grand total / 11. Is that possible?

GST is included in all the numbers above (Shipping & the Subtotal) so while there's discounts taken off the subtotal, in my mind, it just gives me a new subtotal onto which shipping is added and this gives the grand total (I've added the "Grand" just for discussions' sake) and for accounting's sake, the GST should just be 1/11th of the Grand total. But it seems to be calculating in another way and perhaps all the discounts is doing something weird mathematically.

Can anyone tell me what the code would be to edit the ot_tax for this basic calculation? Or is it not that simple?