Due how Brightpearl's templating system works, as soon as you place any code into the template box and hit save. Any subsequent visit to the edit page will alter the template slightly.
Because of this it's impossible for us to see what may be wrong with the template, we'd need to see the raw HTML that you pasted into the template initially.
We have some working template code which you can find here :http://support.shiptheory.com/support/solutions/articles/6000106199-brightpearl-shipping-label-template
Brightpearl's templating system in 'visual mode' will add extra formatting around the HTML, thus if you switch to that view at any point and switch back to the HTML view and hit save, you'll have picked up extra formatting causing the template to become incorrect. This also means that to edit the template at any point after you've saved it, you'll need to re-paste in the HTML code, as by default Brightpearl's template editor page starts in visual mode.