Hi Danika,
Rather than ask for T bar highlights (very dated look in my view), show them what happens when you pull your hair into a pony and ask them to foil it so you're not left with the rooty underside.
They might not be familiar with alternative foiling patterns or they might be very happy to do something suitable but expect to pay a bit more money. I think T bar highlights are usually very cheap?
I work mobile and don't charge a set menu as such but tailor each consultation to the clients needs and give them a price accordingly. With highlights, most of my clients want something natural looking so I do a lot of freehand balayage and tend to use foils mostly when doing pops of colour.