You can no longer generate manifests for past days of shipment without first configuring your "ship date" for a label in advance. This is due to a change in how EasyPost manifests your shipments daily (5:00PM PST) but in a way we can not retrieve using the API.

This means if you've already generated USPS/Royal Mail shipping labels through EasyPost without specifying a shipping date, you can no longer retrieve a paper manifest or barcode for these shipments - there is no workaround.
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If this was a one-off mistake, simply forgo a manifest this time - they are not legally required no matter what your driver says.
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If this was intentional, you will want to work with our support team to implement shipping rules to set future shipping dates for shipments.

e.g. If you never ship on Saturday or Sunday but want labels generated on those days to go out on Monday's pickup you should have a rule to set your ship date to Monday on those days in advance.