Like Jill says, it does depend on the size of your party, which is the most convenient/cheapest way to get from CDG to DLRP.
With a family group, a private shuttle is usually around the same price, or cheaper, than the VEA.
With couples, or families with no little children, or not much luggage, the TGV could suit very well, and can be cheaper than the VEA. The TGV only takes 10 minutes, but you could be waiting around at CDG for your train. I buy my tickets in advance from Rail Europe.
If you choose the VEA, the easiest way to pay is when you board the bus, pay the driver. I've always done it that way.
We have lots of information on all 3 modes of travel from CDG in our Very Useful Information sticky, under "travelling to DLRP by plane"
