Adding a line to account comes at a wrong monthly price.

Those Simple Choice plans are such a great deal!! The new line is only $10 for the talk and text, but things did change a little bit when the data was bumped up to unlimited for everyone. For any line that already existed and had a data feature, they were bumped up to unlimited for no extra cost. For any line that didn't have a data feature or new lines being created, the data feature does cost extra. While you are going through the order process for the new line, did it give you an option for different data features? You might just want to double check which one you selected to see that price difference. The other thing that could be happening is if you add a phone to the order. The extra $15 could be for the device payments or insurance cost.