We have finalized our Croatia trip this coming September. We will need a car for 9 days, driving into Montenegro for three days and then down the coast to Drevnik and onto the island of Hvar for 4 days dropping the car of in Split. Rialtogrl mentioned M.A.C.K and I've been in touch with them. Their price is roughly 41 Euros with insurance included. Drop off fee is $75 to Split airport. Also checked Sixt and the price was similar once you added on the insurances. So basically looking for feedback on car rental companies. I trust Rialtogrl who I believe has a lot of experience in the area but a second opinion can't hurt. Also, MACK sent me a form to fill out and it's asking for a lot of information, Drivers license, Passport number and Credit Card info. Not sure if I want to give all of that info online. Is this standard when renting a car online in anyone's experience.
Thanks for your help

Driving in Croatia was a pleasure. You'll enjoy it. We rented in Germany from Hertz so I can't help with your question. Came here to tell you that Montenegro has specific insurance requirements. If your rental doesn't provide coverage for Montenegro and provide the paperwork you'll need, you can buy coverage at the border. I don't recall exact figures but it seems that we paid US$20 for one day's coverage. Don't know if you can buy multiple days. We drove in from Dubrovnik and the Montenegro border official was extraordinarily helpful and cheerful.