I just finished using one of my characters on Balhae to donate six bags of crafted spellbooks. Each bag contained 100 spellbooks and the weight of the spellbooks in each bag was 300 stones.
I found out that after using the "donate all" option, the bag remained in my character's backpack showing that it held 0 items; however, the weight shown in the "Contents" line didn't reset to 0 and remained at 300/400 stones. I had to either drop the "empty" bag on the floor or in a pack animal's backpack to get the contents weight to properly reset to 0/400 stones.
I found out that after using the "donate all" option, the bag remained in my character's backpack showing that it held 0 items; however, the weight shown in the "Contents" line didn't reset to 0 and remained at 300/400 stones. I had to either drop the "empty" bag on the floor or in a pack animal's backpack to get the contents weight to properly reset to 0/400 stones.