Rates are taking 15-200 seconds to pull or you're seeing errors that the ship server returned a 502 error when using Amazon Buy Shipping.
Shipping labels may take from 15-200 seconds to generate when using Amazon's Buy Shipping labels.
If you receive the rates pop-up after a long delay, you will find "Recommended: amazon took X seconds to return rates, you may wish to disable this carrier temporarily."
SKULabs has documented that only one account will be impacted at a time and we have no associated these delays with any sort of rate limiting on the APIs. Other customers continue unabated while one or more SKULabs customers experience label delays when using the Amazon APIs.
This appears to be service-related troubles at Amazon for their Merchant Fulfillment services (Amazon Buy Shipping), but we're monitoring closely. You can check the MWS health dashboard to see if they've begun monitoring the situation.
Another place you can collaborate with others who are experiencing your problem is on the Amazon seller forums: https://sellercentral.amazon.com/forums/t/amazon-buy-shipping-api-very-very-slow/626011
What should I do?
Our first recommendation would be to disable your Amazon buy shipping carrier temporarily. You only need Amazon Buy Shipping for "Seller-Fulfilled Prime" but you may have other reasons to use Amazon Buy Shipping due to the benefits Amazon provides when purchasing their labels.
Otherwise, our second best suggestion is to wait a few seconds and then try to click the rates button once again. Once you have a rate visible, it should be selected in the rates pop-up to prevent a second request from being required when a label is generated.