The first part of my review is about our experience with Crystal and the second part covers the border crossing to Guatemala and back to Belize.

We rented a car from Crystal as they are one of the few (if not the only) rental company that allows their cars to be taken across the border into Guatemala. Booking and payment was easy over the internet. When we arrived at their Belize City airport facility (in the parking lot next to the other car rental company offices), we were welcomed by friendly and smiling staff. They invited us to help ourselves from their Christmas buffet (cookies, cake, crackers and cheese). Their airport facility is fairly small; later during our trip we met another traveler who rented from their other Belize City facility and he showed us photos from their grand open air facility. We were impressed.
Crystal asked us for our passports and drivers licenses and made copies that the Guatemalan authorities require. Then they gave us 2 stacks of paperwork – one stack for the Guatemalan authorities and one stack for us, with the receipt and notarized certificate that states that we can take the car across the border. Crystal also provided us with a cell phone and cell phone car charger just in case we needed to contact them.
Our rental car was a 2 wheel drive Jeep Patriot and it was in perfect condition and very reliable during our trip. My advice is to spend a little more money and get a car with high clearance – you will be thankful for that considering all the speed bumps and potholes you will encounter in Belize and Guatemala. There are plenty of unpaved roads which Crystal (rightfully) does not allow you to drive on with a sedan. We accidentally took the unpaved Coastal Highway and we would not have made it without our high clearance Jeep. Returning the car afterwards was a matter of minutes. Thank you very much Crystal team!
Long story short: Crystal is a great car rental company and I can only recommend them.

Here are instructions for crossing the border from Belize to Guatemala and back with the rental car from Crystal Car Rental:
Crystal will make copies of your passport and drivers license and attach them to the paperwork that you will need to cross the international border into Guatemala.
When you get to the border, park the car and walk into Belizian customs and immigration building. You can change money to Guatemalan Quetzales in front of the building at the main entrance door or at an officially licensed person who stands next to the first counter that you go to to pay the departure tax. At the first desk show your passport(s) and pay US$20 exit fee and tax per person (BZE$30 plus BZE$7.50 plus BZE$2.50). Then you proceed to the next counter and get your passport(s) stamped. The passenger(s) proceed through the door at the end of the building and wait there for you, the driver. You, the driver, go back to the car and drives it through the checkpoint where a guard examines your passport for the exit stamp. The driver then picks up the passenger(s) and drives through the disinfectant station (looks like a car wash). After the disinfectant station you must park the car and walk to the cashier’s station, give the cashier the license plate number of the car and pay the Q17 for the procedure. You will get an official looking document stating that your car was disinfected.
Then you and your passenger(s) walk across the street to the Guatemalan open air immigration office. You can take either of the 2 lines (the right line is for locals only but nobody seems to care). You present your passport(s) and copies of the driver’s passport and drivers license with the rental agreement that Crystal made for you to the officer. The officer will stamp them and hand them back to you. Then you go to the counter on the left and hand the officer your passport(s) (for all permitted drivers), the stamped copies of your passport(s) and drivers license(s) and the disinfection receipt. The officer will then work on the computer for a long time and disappear for a few minutes. When s/he comes back s/he will ask you to sign 2 documents. The second document needs 2 signatures; you will take this second document to the cashier’s window near where the armed guard is standing and pay the Q160 license fee. You will then bring the receipt back to the other officer and s/he will give you a sticker that goes in the upper left hand corner of your windshield. The officer will intentionally nick one end of it so that you can later take it off easier. You are now free to leave and drive to the bridge. At the end of the bridge is a toll booth where you pay Q20. Keep the receipt as you will need it when you come back.
Welcome to Guatemala.

Coming back, we were stopped at the bridge where paid the Q20 the previous day. We showed the receipt from the previous day and were allowed to pass without paying. Behind the bridge you stop near the Guatemalan customs building and peel the car registration sticker off the windshield. Then you take either of the 2 passport control lines to get the exit stamp in your passport(s). Afterwards you go to the counter to the left, hand the officer the driver’s passport, the car registration sticker from the windshield and the document that you signed when you imported the car into Guatemala. Within 1or 2 minutes you should be good to go. Next, you drive your car through the disinfection station, park it close to the Belizian immigration and customs building and pay BZE$10 at the cashier’s booth behind the car disinfecting station. Now, you enter the Belize immigration and customs building. If you have lots of luggage and are parked close to the immigration and customs building, then you can leave your luggage in the car for now, otherwise bring it with you. In the Belize customs building you go to the second counter, show your passports and get the immigration form(s). You fill them out (as port of embarkation you can put down ‘Guatemala’) and return to the counter to receive your entry stamp. Now is the time to get ALL your luggage from the car if you don’t already have it with you. You proceed to customs behind the immigration counter where an officer will either wave you through or want to check your luggage. Afterwards you exit the building and your passengers wait there. The driver holds on to his/her passport, goes back to the car and drives it through the checkpoint where an officer will ask if you have anything to declare and may decide to search the car. The driver then picks up the passenger(s) and that’s it. Welcome to Belize.