We've been receiving reports that a "Whoa Farmer" error comes up after placing certain items from the Gift Box, and that they disappear after reloading the game. We found out that those items are the ones the players have obtained from the Farmstand posts through the Feeds Panel.
To clear the air, players who help a neighbor through their post should only receive an additional sale for the Farmstand instead of the reward itself. The actual reward will be sent to your Gift Box once you meet the requirements and open the Farmstand window.
Our team is looking into this issue. In the meantime, we suggest that you refresh your game for the sale in your Gift Box to count towards your Farmstand's sale progress.