US postal services is supposed to deliver mail based on the address listed, not in accordance to the name associated with a particular address, historically.
Fact is, mail carriers are discouraged from doing exactly that because people move, yet the address remains constant. One can only imagine the number of deliveries, in error, should the latter occur in an apartment building for example. Although uncommon, return delivery may happen if your a long time resident at your current address. Anticipate this possibility by informing the mailman assigned to your route your brother in-law, cousin etc will be living there for a period of time.
Private PO box facilities vary the number of occupants allowed for one price but most require they are listed on their ledger sheet for that particular box number. Consequently, if you don't formally include the factitious person's name, you MAY NOT receive "their" mail.
Should your source "won't or does't" send product to a factious recipient find another source, because that's utter nonsense, IMO, and IS NOT in YOUR best interest.
Ship a smaller quantity more frequently rather than a large quantity less frequently and establish a steadfast, working relationship with whatever source you choose as PC has suggested.