Miva 10.01 tax change - should I update my custom tax module?

[Mike521w]

NOTE: this is a copy of my post in the Developers / MivaScript category

I use a custom tax module that calculates tax by zip code. It's set up to tax the overall cart subtotal, as well as any taxable order charges like shipping etc.

I was reading the Customer Impact document about Miva 10.01 and it details a change where the built-in modules are calculating tax item-by-item and then adding up that total, instead of using the cart subtotal

Should I make the same change to my module, for some compatibility reason or something?

[Kent Multer]

Module authors need some details on how the new feature works.

From the description given in the Customer Impact Statement, it looks like the only difference in the computed tax will be the result of rounding errors when computing the tax on the entire basket, as opposed to computing the tax separately on each item and then adding them up. Is that correct?

Suppose the basket contains a quantity of two or more for some item. Should the tax module compute the tax on a single item and then multiply by the quantity? Or should it multiply the unit price by the quantity, and then compute the tax on that? A quick look at the latest LSK suggests the latter, but I'd like to get confirmation. Again, there could be rounding errors.

Thanks --