Xero limit API usage per Xero account based on the following number of API calls...
Concurrent Limit: 5 calls in progress at one time
Minute Limit: 60 calls per minute
Daily Limit: 5000 calls per day
Shiptheory typically makes between 1 and 3 API calls per Xero Invoice/Shipment. In the scenario where Shiptheory hits the Xero API limit on your account, the request will (in most cases) be re-queued and retried once the API throttle has released. However, if Shiptheory hits your daily API limit with Xero, a retry will not be queued and you will need to trigger any retries manually from within Shiptheory.
You can avoid hitting the API limit by staggering any bulk processing of Invoices.
It is important to remember that if you preform bulk Invoice updates in Xero, even if they are not related to Shiptheory, Xero may notify Shiptheory that Invoices have been updated and request Shiptheory to download those invoices, resulting in the throttle being hit when you do not expect it.