The best way to pledge for multiple rewards is to add it up manually and put it under one pledge, but you'll have to remember to add in shipping, and Add On for copper. Please see the add on menu for more info on what options are under copper!
However there is a price reduction in shipping as its weight based. I can calculate this for you if you drop me a message with exactly what your looking for!
The other option is to make multiple accounts and pledge each reward under a different account.