Why do so many freight forwarders say they can’t apply Amazon prepaid UPS labels from Seller Central?

I’m shipping from China and want DDP sea shipping to the 5 different Amazon warehouses that is generated in Seller Central. My old freight forwarder just took the UPS labels I paid for and then I would use their USA warehouse address as the pick-up/return address, no problem. Is that what you all do?

Now I’m getting quotes from new freight forwarders and some of them say “I think we must deliver it for DDP” or that they must buy the UPS shipping and I can’t use the labels I purchase from Amazon.

If anyone could briefly share their process it would be helpful. If there’s some reason or law that says you aren’t supposed to use Amazon’s labels this way that would be good to know too!
Thanks for any information!