Cart Rule to exclude tax

[batman42ca]

I have a client that wants to be able to create a Cart Rule (a voucher) so that when the voucher code is entered, the contents of the shopping cart are tax free.

 

Anyone want to suggest a cost for modifying Prestashop to permit this type of cart rule (if possible in a way that will survive an update of Prestashop)? Or does anyone know of a module (free or for purchase) that would already do this?

 

I'm a PHP programmer but I don't have time to learn what I need to know about Prestashop to do the work myself.

[Enrique Gómez]

The simplest solution would be create a voucher with the percentage  to nullify the tax..

 

For example I have 21% VAT then 1-(1/1.21)=0.1736

 

If my client purchases are 12.5€ cart then with VAT they would be 12.5 + 0.21x12.5=15.125

 

with the discount 0.1736*15.125=2.6257

 

15.125-2.6257=12.5

 

But as far as i know the voucher does not apply to the shipping costs

[batman42ca]

Thanks but from what I think I've seen, unless I have it configured wrong somehow (hopefully I am wrong) it actually works like this using some very easy numbers:

 

product cost = $100

10% discount = $10

subtotal = $90

10% tax = $9

total = $99

 

So I can't just use a 10% discount if my taxes are 10%. Is there some configuration option I can use to fix this?

[Enrique Gómez]

Your discount should be 1-(1/1.1) =0.091 = 9.1%

 

product cost $100

 

9.1% discount =$9.1

 

subtotal =$90.9

 

10% tax =$9.1

 

total =$100

[batman42ca]

I had been trying to derive that equation myself but kept tripping up for some reason. It would work fine if not for rounding errors.

 

Here's a real example:

 

product cost: 599.99

tax rate = 13%

discount = 1 - 1/1.13 = 11.504424778761 ... BUT prestashop will round this to 11.5

 

subtotal = 530.99

13% tax = 69.03

total = 600.02

 

So, it's close but not the same as truly excluding the tax.

 

UPDATE: seems my client is content with the improved math so for now, I no longer need any special code for this. Thanks for the help Enrique.

Edited March 24, 2014 by batman42ca (see edit history)