re-using boxes

[robinsgirl]

i need to send some things back to the states (nothing major or fancy) and i was hoping to just reuse boxes that have been sent to me. does anyone know what the royal mail stance is on this? do they have to be devoid of markings, or do i have to black out things out, or is it enough to just make sure the "to" and "from" addresses are clear? i know usps sometimes gets cranky about things like this, but i can't find answers on the royal mail website.

thanks!