Insurance offering way less than my car is worth after total loss - help!

Got rear ended last week. Other driver's fault, no dispute there. But their insurance is offering me $8,500 for my car when I've seen the same year/make/model selling for $12-14k in my area.

They're using some valuation tool I've never heard of. Wont tell me exactly how they calculated it.

What can I do? Can I negotiate? Should I get my own appraisal? This was my only car and I need to replace it.

Im in Texas if that matters for any state specific rules.