see your problem here, but they have given you a option.
what would you prefer, them organise a pick up time, then you have to make sure your in at that time (meaning the pick up time they have organised could be when your at work or something so you then have to take a days holiday, they dont know your schedule...). there not going to send out a replacement part untill the wrong part has been sent back to them, pretty standard for internet shops, otherwise i could order something say theyv send the wrong thing and then they post the correct part out and i dont post the incorrect part back, therefore 2 parts for the price of 1, one part i need 1 part to sell on ebay.
i get where your coming from, its a bit of a chew on having to organise stuff, but the easiest way...
IMO, stop winging and sort out the courier to get the pads back.
bet you have a printer at work that you can use to print out the label, and bet you have a mobile with unlimited/loads of minutes, i seriously doubt that you use within 5 mins or your total allowed mins on your contract every month, therefore chances are whether you make the call to UPS yourself or not isnt going to make any difference to the price of your mobile phone bill...(unless its a number that costs? in which case egnore that last point)