By adding a border to your artwork file yourself you have better control over the final size of your print. This is important if you want a standard size print to fit into a pre-made frame. For example, if you are ordering an A2 print for an A2 frame and want a 10mm white border around your image, add a border inside the dimensions of the A2 area. This will mean your image will be smaller than A2. It will be 277mm x 400mm.
We suggest using an editing application such as photoshop to add your border, make the dimensions of your file A2 with a white background and reduce or crop your image to fit to border size you want.