Issue-9231 include promotions into order taxes calculation

[sacha75]

See Issue 9231
This workaround will result into correct tax calculation at order level with order adjustments.

[squash-labs]

Manage this branch in Squash

Test this branch here: https://sacha759231-taxes-not-includin-0zj6a.squash.io

[MatthewKennedy]

Hi @sacha75,

I was looking at this by copying your code across into a version of Spree 4 I have locally, and I can't get the correct tax.

Can I just check the values I get with your results?

I added 2 items to cart, $100 each with a $50 whole order discount.
1st item is taxed at 20% (tax included in price)
second item not taxed.

I get tax at $16.67 when I go to checkout.

Expected tax of $12.50 ($100 USD item -$25 = $75 tax in price $12.50)

[sacha75]

Hi @sacha75,

I was looking at this by copying your code across into a version of Spree 4 I have locally, and I can't get the correct tax.

Can I just check the values I get with your results?

I added 2 items to cart, $100 each with a $50 whole order discount.
1st item is taxed at 20% (tax included in price)
second item not taxed.

I get tax at $16.67 when I go to checkout.

Expected tax of $12.50 ($100 USD item -$25 = $75 tax in price $12.50)

Hi @MatthewKennedy,

Thanks for your feedback, I had a wrong condition and I have commited an update.
On my local version of Spree 4.2.0 it now gives me correct result for your example.

I'm trying to get the test passed on my local version then on Circleci, I'll keep updating this PR until it works

[sacha75]

I changed it to work only with included_tax_total as calculation should be different for additional_tax_total.
I believe that business logic is the following: order level adjustment is intended to be with or without tax depending on items being with or without tax.
If Item A is $100 tax included, order discount of $10 is expected to be tax included. We expect to see at the end of the bill $90 and see the according tax amount (B2C logic)
If Item B is $100 with $10 of additional taxes, order discount of 10$ should apply to item net amount. We expect to see at the end of the bill $90, then the correct tax amount to add, then the final total (B2B logic)

Spree allows a mix of included and additional tax, in that case there is no way to calculate correctly the new tax. One option would be to add to order adjustment an information to know if user intended adjustment to apply it to the amount with or without tax.

On the other hand not fixing that issue results into incorrect tax calculation which is a problem for e-businesses.

Before going any further I would like to have the opinion of the Spree team

[MatthewKennedy]

Hi @sacha75

Have you tried this extension?
https://github.com/MatthewKennedy/spree_fixed_amount_discount

The extension works for mixed cart of taxed and none taxed items.