I just typed this out, thought it was informative and decided to post it here

The straight hair comes in 14" 16" 18" and 22". Get a tape measure and
see where you would like your hair to fall.
Hold the start of the tape measure at the nape of your neck, above your
ear and near the top. The hair becomes naturally layered at the bottom
with the application.
There are different grades of hair and the price varies greatly. Most
of my clients go for the least expensive hair which is really nice,
not "cheap" hair but not very expensive. I wear it myself. The price
difference between the least expensive and the most expensive can be a
few hundred dollars.
If you choose the more expensive hair it CAN be reused but would need
to be shipped back for re-tipping. The less expensive hair can't be
reused.

I recommend that the extensions be redone every 3 months to prevent
your own hair from being knotted at the roots (since that part can't
be brushed to shed the dead hairs)