I know on p4d if you send a mkl burner @ 21kg in a standard bike box 170cm x 80cm x 25cm. It works out at £8.79+VAT [the new price after the recent hike] But bearing in mind most bmx's weigh an average of 15 - 18kg you ahould have no problem, just keep the box as small as you can, as its normally the cubic cm's that bump up the price rather than the weight, some fettling of dimensions is sometimes required
