**Steps to reproduce:**
- Install Accounting, Sales and account_avatax_sale
- Configure AvaTax in Accounting settings
- Create a product with an AvaTax Category (e.g. [D00000000] Digital goods)
- Create a SO:
  * Customer: [US customer] (e.g. Azure Interior)
  * Product: [the created product]
  * Unit Price: [any] (e.g. $ 200.00)
  * Fiscal Position (Other Info tab): Automatic Tax Mapping (AvaTax)
- Confirm the SO => The taxes will be recomputed with AvaTax (e.g.  $ 20.50)
- Create a down payment from SO (via "Create Invoice" button):
  * Create Invoice: Down payment (percentage)
  * Down Payment Amount: 50%

**Issues:**
1) There is no tax on the down payment
2) When confirming the down payment, AvaTax taxes are computed from the Down payment product.
It should not because the Down payment product may have a different AvaTax category than the product on the SO, which may lead to different computed taxes.
The taxes on the down payment should be the same than the ones on the SO.

**Cause:**
1) In "sale_external_tax" module, the "tax_ids" field is removed when preparing the invoice line (in "_prepare_invoice_line" method). Even though "account_avatax_sale" module depends on "sale_external_tax", we want to keep "tax_ids" when using AvaTax.

**Solution:**
1) Create a method checking if taxes should be removed in "sale_external_tax" and override it in "account_avatax_sale" module to keep them.
2) Exclude down payments from external taxes recomputation.

opw-3971513