UK Composite Rate Challenge

I need to customise Zen to allow tax rate to be selected by looking at product and customer information to allow use of composite vat rate.

eg

Product A purchased alone = 17.5% tax
Product A purchased alone by Registered disability customer = 17.5% tax

Product B purchased alone = 17.5% tax
Product B purchased alone by registered disability customer = 0% tax

Product C Purchased alone = 17.5% tax
Product C purchased alone by registered disability customer = 17.5% tax

Product D Purchased alone = 17.5% tax
Product D purchased alone by registered disability customer = 17.5% tax

Product A and Product B purchased = 17.5% tax
Product A and Product B purchased by registered disability customer = 6% tax

Product A and Product C purchased = 17.5% tax
Product A and Product C purchased by registered disability customer = 6% tax

Product A and Product D purchased = 17.5% tax
Product A and Product D purchased by registered disability customer = 17.5% tax

17.5% is standard tax
0% is available on certain products for certain customers otherwise it is 17.5%
6% is company composite rate when combining certain product types with qualifying additional products.

So i need to add info to customer to store if they qualify for reduced tax rate.
Info to products to say IsType A or IsTypeB or ...
or could flag as IsTypeA, Qualify0, TriggerOnly.

Two questions:

1. Is this possible without completely hacking Zen about and having to rewrite it every time there is an update

2. Where do i start!?

Hi,

First just some clarification.

You say that on certain product combinations that a 6% rate is applicable. I assume that this is not a 'true' VAT rate, but some form of discount that is applicable only to your company ?

If that is true then you would need to actually apply the correct VAT rate for accounting purposes and to keep HM Customs happy, the difference (11.5%) would need to be applied as a discount. For this you would need to write an order total module (see something like the group discount module for inspiration).

For the other combinations which I am assuming are based on standard VAT regulations, this is still doable, although i'm not 100% sure its doable on an override basis.

I would start by looking at the cart class, there are some notifiers there that could possibly be used.

What you want to achieve (at least at the cart level) is change the tax class/rate associated with one or more products.

There are some examples in the wiki related to adjusting cart details based on the contents of the cart.