I can only make a reservation online, there is no option to make a payment. Does anyone know why? I hope this isn't done in order to raise the fee when one actually shows up to rent the vehicle?

Use rentalcars.com or rentcars.com

There never is. I've never rented a car and had an option to pay online, or before arrival. Probably because they need your drivers license, etc. You give them all that at the rental counter when you get there. They do, now, also have kiosks where you can self-serve, too, right at the rental counter. I've never gotten a price increase after reserving online, but sometimes had a reduction.

No price increase