Pre paid shipping is not required, and if so, this would indicate that you have a space.
you usually pay the shipping on arrival and clearance.
Someone is ripping you off..
If you have a good broker, they will be able to fight to get your cars on.
Last Nov I bought a 300Z and was told no room. After raising this with my broker, suddenly there was room!! Apparently they keep alot of space for larger shippers so a one car shipment is very low priority...
This is what your broker is paid for...to ensure the best possible outcome, not to take your money and then leave it till 'whenever'
Best of luck