Build the channel definition map

Amazon reports gross before returns.

Before you use the output

• Verify the return figures came from actual returns reports, not from a category assumption. This is the largest single line and the most commonly estimated.
• Check the GT contribution of 96%. It almost certainly excludes distributor margin already given as a lower price — if so, the comparison is not on a like basis.
• Confirm advertising attribution. If brand-level ad spend supports marketplace and own-site sales, attributing all of it to one channel distorts both.
• Count the distributors making up GT revenue before treating it as the growth channel. High contribution plus high concentration is a different risk profile.