Everyone probably knows this, but I didn't until Saturday. So, I thought I'd share!
Your garrison quartermaster, , sells "missives". These missives, when turned in, reward you with 800 apexis crystals. They cost 200 garrison resources to purchase, and are a quest (so you need space in your log).
What I didn't know is that you can "stack" these. So, if today's daily is Darktide Roost, you can buy the and receive 1600 crystals total for doing the same work as 800 would ordinarily reward.
The missives also stack with Garrison Campaigns. Also, you can do them over and over, as long as your garrison resources support your purchases.
If you are after the followers, pets, mounts, gear, or bonus rolls using these crystals, this is a way to quicken that pace.
I used them on Avelmi for the daily, then a garrison campaign so I could get her into LFR quicker.