Thanks for letting me know about this. I can see here that on the line you refer that promotion was attached on it.
 
For your device you will charge for $45.84 however you received a promotion of $32.50 and the remaining charge for  the device was $13.34 only the promotion as intact and you received the promotion that was offered to you.
 
In order for you to understand how promotion works let me share you an example.
 
Since you trade in an device to get this promotion the value of your trade in was less to the promotion offered. For more visibility of this example you may refer to the sample below.
 
Example: Trade-in an iPhone 11 Pro = $160 FMV. $1,000 offer value - $160 FMV = $840 in bill credits (approximately $35.00 per month).