Issue rounding return total if you return several items and order has discount

Description

Prerequisite: An order with 3 items SKU-31957691 (price $171.50), an order discount of $50, and a sales tax 9%. The order total excluding shipping is 506.31 USD.

Steps to reproduce:

1. Return 1 item SKU-31957691 --> return total is 135.43 USD.
2. Return a second occurrence of that item, SKU-31957691--> return total is 185.43 USD.
3. Return a third occurrence of that item, SKU-31957691 --> the return total is 185.43 USD.

Expected:
Return total should be 506.31 USD

Actual:
After 3 returns, the return total is 135.43+ 185.43+185.43 = 506.29 USD