Unable to pay late payment.

When I go to bills it brings up that I owe $23. When. I click to pay it it brings me to the mobile version and the price is set to $5. When I go to pay it I enter my new debit card info in. As my old on got lost and just now got around to updating the new card info.

I try to process a payment. It gives me a error and goes into mobile version of billing. Saying I owe $18 then I click make payment then it says I owe $300+. When I try to pay the initial $18 and  the $5 late fee. I'm guessing. I try processing the payment again and gives me the same error. I'm not wanting to spend pay off all $300 I'm trying to get the previously owed amount paid off so that it can go back to charging me like it normally does.

This is for a phone that I was told would be free. Making excuses like it's his birthday and this will make his day. I was never told I had to activate it with in a week or two for it to be free or I never would have agreed to get this phone. I then explained what happened to a manager or some high up person.

He then activated and deactivated and told me he needed to do this in order to lower my bill. So I let him and it did.

He said if I take it to a store they will refund what ever I paid into this phone.

I would prefer if I can mail it back instead that way I'm not waiting months or years to go to a Xfinity store that's over a hour away while unable to drive.

My question is can i get it mailed out of have my billing situation fixed untill I can mail it or visit a store.