You can get free sentries delivered from the supply shuttle without any points being spent. The drawback is that you can no longer order anything new from cargo ever again. But hey, free sentries amirite?
Steps to reproduce:
1. Make sure the shuttle is going back to central or is waiting there.
2. (Optional) Accumulate a lot of points, 800-1000 will do. Order and approve a number of sentry crates, explosives, etc. making sure to keep 30 points left.
3. Order a single collector crate and approve it. Note that this has to be in the same shipment.
4. Call the shuttle back. You'll receive the ordered things without the collector crate.
5. Ping-pong the shuttle back and forth. Every time it will bring the shipment except the collector. In fact, since the 'crates' are free you'll actually start gaining points back rather quickly.
6. Cargonia triumphant!

I'm not sure what happens if you order a new shipment with another collector crate at the end. It might be possible to 'reset' the free shipment but I haven't tested that yet.