I’d be questioning or at least challenging on the API call rate. Get the view on how many calls they actually need today. Even if on the smaller plan giving 2 calls per second that’s still quite a lot for a lot of merchants when it comes to the amount of orders being created and updating a property afterwards. You might find it’s not really a problem at all to make a call to get latest data set.
There’s also the option of storing the attribute current externally if there was a super strong desire to avoid an API call. So order is created with attributes in place, that info can flow to a DB via webhook, and you pull the data from that db instead of making an request to get it.
Are attributes the right place for this to live? Perhaps you might get them as attributes to start with but for the final choice perhaps you update a metafield instead. It’s an API call either way.