Gateway Not Returned Fee

I'm being charged $150 for not returning a gateway.  The fee is being applied to my wireless account.  The wireless account was opened in my name with my credit.  The internet account the gateway is associated with was opened in my dad's name with his credit.  Before he died I merged the accounts online so I could manage them with one login.  However, ATT switched my wireless account to my dad's name and now I'm being charged for his fees.  I've dealt with enough of his creditors to know that this isn't right and I want the fee removed.  

The thing about it is I don't mind returning the gateway, but ATT is being unreasonable.  The ATT rep I spoke to in March told me to return the gateway by taking it to the nearest UPS store.  I told her I couldn't do that because the nearest store is an hour away, and I don't have reliable transportation.  I've read that other people received smart labels to return their devices.  I never received anything like that.  If they wanted the gateway back so badly, they should have sent a return kit.  I would like this investigated and the fee removed.