Re: Tread Depth
If they will mount them and balance them for 100 thats a decent deal.
tires should be replaced at approx 3/32nds so you are at about 2/32nds of usable tread on those tires (realistically many people run at least one tire down to 2/32nds or less. A new tires is going to be in the neighborhood of 12-14/32nds for a highway or all terrain tire or upwards of 18 for a MT.
using the numbers given your looking at about 18-22% of usable life left in the tires. (2/32nds into 9 and 11/32nds to account for the last 3 at the wear bars).
Just make sure you call a tire shop if these guys wont do it for you, verify you can get a tire shop to do it for you, I would imagine some retailers/installers wont want to touch tires approaching the end of their lives.