Insurance offering $1k less than repair estimate

My parked car was hit by a Uhaul, Uhaul's insurance RepWest claimed liability and based on the photos i sent them they estimated $1400 in repairs. The adjuster i spoke to was ready to cut me a check but i told her i was scheduled to get an estimate myself the next day and she said no worries we'll wait until you get the estimate. The next day I got my own estimate from a shop approved by Mazda and also in my insurance company's repair network, and they estimated $2400. I sent the estimate to RepWest and suddenly they said since their IA already did an estimate they could only issue a check for the $1400 and the rest would have to be submitted as a supplement. Don't understand why she was willing to wait for the estimate yesterday if she's now saying there's nothing she can change until i put my car in the shop.

My hangup is that i believe supplements are meant to cover things that are discovered once work begins (eg. we took the bumper off and realized that the sensors are damaged) not just a discrepancy in the cost to repair which is what this is. Feel like i'm being scammed into taking the $1400 and then they'll just reject the additional cost later because its not a new discovery of damage just a disagreement on repair cost. The repair shop deals a lot with insurance, but when i mentioned RepWest they'd never heard of them so there's no relationship for them to maintain and not sure how much haggling they would do with the insurer on my behalf.

Alternatively, i could make a claim through my own insurance which has a $500 deductible, but then i have a claim in my history. OR, i could take the $1400 and then take the Uhaul owners to small claims court (they're my new neighbors so i can contact them easily) for the balance due (i assume this is an option?).

Thoughts on best next steps?