HSN keeps changing my shipping address

We get our mail at a post office box. In order for the post office to correctly handle packages sent via UPS SurePost when the post office makes the actual delivery, the post office told us to use a shipping address similar to “1234 State St. #789” where “1234 State St” is the location of our house (and suitable for UPS Ground shipments and similar), but the “#789” part triggers the post office worker to try to match my name with the name on PO box 789. This system works perfectly with QVC, ShopHQ, Amazon, Target and many others. But not HSN. When placing my order, the shipping address is shown correctly, but as soon as I confirm my order, HSN strips off the “#789” part. Then the UPS SurePost package gets to the post office which doesn’t know what to do with it (since they don’t deliver to that street address) so they return it as undeliverable.
We call customer service who confirms that the #789 has been stripped off my new order. They cannot add it to the existing order, but they do put it back onto the shipping address on our account. But on the next order, it gets stripped off again.
If all the other companies can do this correctly, why can’t HSN? Very frustrating to find out the package you were waiting for got returned as undeliverable because HSN changed the shipping address.