OP:
KBB list your car, private party value, between $4900-$6100. That is a 1999 XLS with 100k and good body. I did not select any option, just a bare 99 XLS trim.
150k miles is between $3300-$4500
What will happen if it is totaled is they will low ball you. They will call and say your car is worth $2900; where can we send the check... What you say is you think that is a little low do you have a fax number or can I e-mail you local listings showing a greater value.
Now you need to go to autotrader and craigslist and find all the 1999 Avalon's you can that are priced at the upper range. Send the highest 3 Ads to the adjuster, try to have at least 2 of them be from dealers. They will then come up with a better number. Might only be a couple hundred more or maybe another k. Either way its more money for little work.
Good luck and update as it plays out.