We've had A LOT of customers ship their packs with full canisters and haven't heard of anyone being hassled for it. However, if you were to get caught, then there could be big fines. As a commercial shipper, we have been audited by the Feds to make sure we have the right training, right paper work, etc. (and I get a call about 2-3 times per week from the HazMat auditors).
To do it legally, you may try a local UPS or FedEx shipping store and they can ship it for you. If you want to label it correctly, you need to have a Green HazMat placard sticker on the box and label the box with UN1066.