I just bought single day tix thru the DLR website for a day trip in 2 weeks. They arrived two days ago. I wanted to save myself from having to wait at the ticket booth in the morning, because time will be tight.
If you have a comfortable 10 days, they will ship for free. I paid $10 for express shipping to make absosulety sure I got them before I left for California. They arrived after about 5 days.
The ironic thing is that it is difficult to find places that will sell you single day tix, except for DLR directly - or the DIsney stores. If you are doing 2+ day tix, there are a lot of non-DLR options for purchasing - I bought 4 day tix thru Ares Travel in 2005 at a discount.