Just tell them that you'll only accept pickup from your own house unless they send you a cheque for postage costs. It's not like you're losing out in the meantime as you had to pay 50 pounds to NOT use their service.
If they have an e-mail address send them pictures of the modem, next to hammers/the kitchen sink hinting that it may get damaged in storage, or alternatively you could do some programming :)
If they have an e-mail address send them pictures of the modem, next to hammers/the kitchen sink hinting that it may get damaged in storage, or alternatively you could do some programming :)