USPS Priority flat rate boxes $5.80 for 315 drive links or less, $11.60 for 630dl or less, and $12.75ish for more.
Stopped by a local Kinko's/FedEx Office store the other day, and noticed that they have 'Flat Rate' shipping too now. I don't think that you can beat the USPS Small Box Rate, but it looks like you can get a whole lot more into the FedEx Flat Rate boxes, at the next step up, for less $$. Might want to check into that if there is a location near you.
FedEx is usually pretty reliable too (except for occasional, large, flat screen TVA!).
Philbert