If you have the time, apply for a credit card now.
It will probably only come with a limit of a few hundred dollars, so use it frequently, but pay off the balance each month so you don’t accrue interest.
After a while the limit might get raised automatically, or you could ask the bank to raise it. I don’t know what sort of limit the car rental companies would want but a couple of thousand should be enough.
After your trip to Sheris, you can either cancel the card, have the limit reduced or just stop using it.
Renting a car is the best way to increase your spending power at Sheris.
Using the prices quoted here – http://www.sherisranch.com/adult-sex-vacation/ – and Maverick’s estimation that the limo driver/company gets 30% (from this thread – http://newblog.sherisranch.com/forums/topic/question-about-shuttle/ ) that $2000 voucher is actually only equivalent to $1400.
