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I am looking at an item on eBay which ships via, "USPSFirstClass". As per the GeteBayDetails API, Shipping time for this service is 2-5 days. But the View Item page specifies the "Estimated delivery time" as 5-8 business days. Why does this discrepancy exist?
Estimated delivery time
This is not a discrepancy.
The estimated delivery time includes the shipping time + whatever time is required by the seller to deliver the the item to the shipping carrier.
Thus estimated delivery time is based on the seller's handling time (specified via DispatchTimeMax in AddItem), the shipping service selected, and when the seller receives cleared payment. If the seller has specified a handling time then the estimated delivery time will not be equal to the Shipping Time of the carrier, as you are seeing in this case.
See Handling Time and Estimated Delivery Time for details. (If link does not work, you can go to the Trading API Guide shown in the additional resources, and search for "Handling Time and Estimated Delievery Time" and it is under the Misc Shipping Topics)