3 weeks prior, I replaced the AC compressor and alternator. Hopefully that will help to increase the offer.
It won't. If the AC was non-functional then you would have made out on the positive, with FMV assuming that everything worked and that it was in good condition. But you ended up getting screwed on the deal - sorry. You also won't generally get paid for upgrades, such as stereo, wheels, etc. but you can often swap those out of the vehicle remains haven't been sold yet.
The advice given here before is correct - do your best to find comparable vehicle listings on Craigslist, AutoTrader, etc. in your area if possible. Since we all know that vehicle listed prices tend to be unrealistically inflated, you will probably be able to find more favorable comparables than what they offer you. Just keep in mind that their obligation is that you are made "whole", meaning that they compensate you enough to buy a comparable vehicle in the same condition that you previously had.
Good luck. My car was totaled several years ago - it was a minor accident but an old car. I had previously been bumped while sitting still about a year before and I pocketed the $800 reimbursement instead of having it fixed (mostly just bumper scuffing). So when it was totaled I came out pretty well, especially since the car had 220K miles.