Charters are Duration-Events. Or Projects with an expiration date.
When you buy a Charter, put your cube on it (similar to a Project) and follow its instructions. That might be this turn, but the cube is there to keep track of the fact that you bought it. This allows for "next turn", "until then" and one off "when you" mechanics. Unfortunately, this also means you can't buy it multiple times at once. When you have done all the things the Charter asks you to do, remove your cube, and you can buy it again.
Here's some modified official events (I used the Duration colour for these mock ups):
Expedition is now safe from Militia and friends. You can choose your Summon when you're ready to play it, rather than when you buy it.
But you can do more with Charters, for example (analogue of sorts for Treasury):
You gain the Silver right away, and as long as your cube is on the Exchequer, you get +$1 at the start of your turns. Once you gain a Victory card, take your cube back.
I'll judge based on how much I'd look forward to games with the Charter. The design should make use of the fact it's a Charter, but I'm fine with simple "next turn" designs where there might be some way to do it as an Event.
In terms of naming, I had in mind promises, or contracts, or "charter a boat". But I'm not going to judge based on names.