1. There are flat fees per unit for removal and disposal orders. The fees are typically based upon the Size Tier and weight of the unit. As far as I know, the shipping costs are included in the flat fee. The fees are very reasonable. But yesterday, Amazon announced a promotion that waives removal/disposal fees. So you should be able to send the 11 units to your sister’s place for no cost if you act quickly.
2. Amazon will resell any returns that they deem to be fulfilable. This is sometimes problematic as Customers may return a damaged or incomplete product and then Amazon resells it. Obviously the next customer is then unhappy.
You do not have to do anything for Amazon to resell your returns. They do it whether you want them to or not. In fact, it is impossible to turn off.