It takes that long because they have to wait to ensure the label isn't used after all.  They have only your word for it that you are not going to use the label, and if they refund your money immediately, and you then go ahead and send a parcel using that tracking number they are out of pocket.  The cancellation process to void the tracking number probably takes a day or so, and in the meantime if the label is used, it may well slip through the system.   Besides, you are not actually paying out the money immediately, it is simply being added to your Ebay account and isn't due and payable till the next monthly billing anyway, so in many cases, the refund can arrive before you have to pay.