Got an answer from Amazon - for what it’s worth - as follows
Hello from Amazon Selling Partner Support,
Having read your email, I understand that you would like to investigate us about the slowness of the UPS carrier. I will certainly help with this information.
I’d like to inform you that this type of slowness occurs only because of covid 19 issue. Once this issue have been fixed then it will com back with the normal process.
Since you are using Amazon partner carrier, we will take responsible for deliver and receive the parcel, the parcel might be delay in delivery due to some external facts. There is no further active from seller end. Once the units received, we will start the receiving process.
Most products will be received within 14 days after the entire shipment has been scanned at an Amazon fulfilment centre. However, some shipments can take longer to complete if issues are encountered during the receiving process or if products are reshipped to other locations in order to store them closer to customers.
Waiting until that date helps ensure that the products in your shipment have had sufficient time to reach the appropriate fulfilment centres, including your products that need to be reshipped to other locations in order to be closer to customers. Kindly wait till the target date so that your units will be shipped / delivered.
For more information about Reconcile your shipment verify the below link:
Please let us know how we did.