I have noticed that some Square API POST calls have some amount of latency before the GET seems up to date… for example, the CreateCard endpoint: If I call create card, it returns successfully and the Square API returns the created card. However, it is 4-5 seconds before the List Cards endpoint returns a list of cards that shows the new card (any calls before that show the “old” list of cards). The question is - during this period, is the card able to be used? What would happen if I tried to make a payment with the newly added card in this 4-5 second period? Is the card actually in the system, but it just takes a few moments for the List Cards to return it?
The newly created card will be available to charge once you get the successful response after you created the card. Honestly 4-5 seconds seems a little long for when the card is returned in the list response. We do have some asynchronous functions that may cause a delay but it’s not more than just a few seconds. What’s your application ID and do you have the time when you saw this behavior? 
App ID: sandbox-sq0idb-GSYjVJtwxPQjaeMaEKb2ZQ
As far as time goes, I’ve been developing for the past couple of weeks (I did some testing with this particular feature yesterday, adding and removing cards).
Also: I’m assuming I can post my application ID on the forum, and that it’s the access token that’s secret?
Great, thanks! Yes, the access token is the secret and you should never share it.