Gift box - Valentine's day - 0.4mm

Set first layer infill pattern: Concentrinc, Archimedean chords or Octagram spiral
It's important to set some non-paralel pattern. There is risk of breakage at the groove.

For the top layer set some symetric also: Concentrinc, Archimedean chords or Octagram spiral - because it looks better.

Bend in the grooves immediately when removing the product from the plate while it is warm. Or you can do it in hot water to reduce risk of breakage.

applicable for PLA:
Deeper grooves for bending - so that the model is only one layer thick at the groove. 
Added grooves for hearts.