You said you bought a new phone, I'm assuming you did not put it on a new tab?
You also said that you paid off the small outstanding tab, how much was that?
Say you're on a tab small, and you get $10 monthly tab installments. Say your remaining tab balance was $50. Then it'd bill you $10 for the installment, plus the buy-out at $40 (balance minus installment).
If you took a new phone on a new tab, then you'll get a tab charge for the new tab.
Also, how did you pay off your Tab? You have to go through >login to self serve > View Tab balance > Pay ff Tab
Just adding remaining Tab fee on your bill doesn't process as "Pay off Tab".