Shipping invoice question

Hey guys, hoping to send off my first haul to the UK in the next few hours or so, and was just wondering about how the whole invoice thing works. Once you get to the end of the shipping process, you get the box that asks about additional information, do I need to ask my superbuy agent to attach an invoice on the outside of the box, showing what's inside and the value, or will they automatically do it showing the total that I declared?  As I have opted to remove all of the boxes and tags, is it also worth asking them to mark the package as second hand clothes, so the declaration of $17 looks more real?