Comcast xfinity added fees to bill with were different then what they said it costed! Atlanta Georgia

Comcast said more then once what it cost for a comcast worker to come to my house to put in a new cable box!  The cable box is the X1 and you install it just like the rest of the boxes they mail to you to install yourself!  The price was $39.99 to install their equipment!  I checked my bill and they have a $20.00 fee that I asked about and the $39.99 fee!  No one could tell me what the $20.00 fee is for!  They put things on your bill like other charges and don't explain what the charges are!  On top of that they bill customers in advance!  I can't watch tv in advance!  That needs to be stoped!  When they bill me in advance my bill is over $300 or more!!!