product white blends into background white

[denzjos]

Draw a path all around the box the 'Path Tool' (straight lines). 
- Select / Select from Path
- Select / Invert
- Select / Shrink (1 or 2 pixels)
- Select / Feather (2 or 3 pixels)
- delete the selection 
You can straighten the box with one of the 'Tools / Tranform Tools'
Optimise with 'Filters / Enhance / Sharpen'