A few more things...
Alamo Rent A Car had unlimited mileage and free drops in different cities. Check the travel aggregators for a rate and then check Alamo directly. It was a good bit cheaper.
Book the smallest car you can comfortably fit into... They'll put you in a bigger car anyways. Fuel is expensive and in the Highlands/Islands range is your friend. Not to mention the roads are laughably tight.
Here's a cute one... the speed limit signs don't have the unit of measure on them, being in the UK I figured it was KPH, and was careful around traffic cameras and in small towns, then I noticed the car's Odometer was set to MPH... So I was pissed. I mentioned it to a local and apparently the speed limits are in MPH not KPH so all good... no tickets in the mail for me... suck it Italy!
Consider the full insurance protection plan, if going into the highlands... According to the lady at our car rental return 75% of the cars coming back from the Highlands need at least 1 new tire. (seemed outrageous, but she VERY carefully checked all the tires) When asked her about it she said UK rules for rental companies are very strict and ANY gouge of the sidewall has to be replaced. I drove pretty gingerly and still hit plenty of stuff on the sides of their narrow ass roads.
You'll want more times on the islands than you think.