You can easily place your pattern on a box via smart object in PSD file. Just double click on a smart object and place your pattern. Ideal for light coloured patterns or patterns with transparent/white background.
You will get 1 PSD file with smart object, 1 JPG file (high resolution) with a box without any pattern and 1 seamless pattern with a christmas trees (PNG with transparent background included also!)
This mock-up is made from real photo I captured myself.