We made the mistake of setting up combined billing without being warned that ATT will punish you in little ways for doing this. It is not worth the 10 discount to do this!!
In our case, we wanted to replace a cell phone. We wanted to pay off the 3 months of payment left on our phone and then get a new phone with the Next plan. However, if you have combined billing ATT will NOT let you do this. We tried to do this at BestBuy and at an ATT retail store. Both took hours (literally) to figure out that this can not be done.
How do we reverse the combined billing quickly? Perhaps better would be to ask how to switch to Verizon since ATT doesn't seem to want us here. We have been customers for 16 years with a bill over $500 per month and they still don't seem to care at all.
The customer service people are very friendly and nice, but they just don't seem to have any authority to actually solve problems.