Shipstation has set specific rate limits in order to "ensure consistent application performance and increased scalability." The current Rate limit for each set of the API Key and Secret is 40 requests per minute. Once you have reached the limit for the minute, you won't be able to make more requests until the time resets at the beginning of the next 60 seconds.
Questions about Shipstation? Email us at firstname.lastname@example.org or chat in on the site.