I suspect most graphics card boxes should accommodate a 7600XT you may just want to add a small piece of extra foam to take up any unused space if it was originally for a much longer card. A box for a 6600 or 7600 should fit nicely.
You can ask your card manufacturer tech support for clarification on specifics of the shipping container, but I suspect they just want to ensure you don't throw the card in an empty box where it will bounce around (and possibly be damaged) in shipping.
Ryzen R7 5700X | B550 Gaming X | 2x16GB G.Skill 3600 | Radeon RX 7900XT
Ryzen R7 5700G | B550 Gaming X | 2x8GB G.Skill 4000 | Radeon Vega 8 IGP
Ryzen R5 5600 | B550 Gaming Edge | 4x8GB G.Skill 3600 | Radeon RX 6800XT