Sincere information for maximum effectiveness.

Distance is important UV radiation diminishes exponentially, so 10 inches is not 1/10th of the power it is at 1 inch from the source, it's almost non existent. (that's why 30 minutes in a box)

I would line your whole box with foil (shiny side out, rub it to polish it a bit if you can)

If the UV light cannot touch it it does not sterilize, you can shine UV on a box and if 1 side is in the shadow even for many hours, it will never sterilize. Make sure to flip your packages. (put a rack in to allow reflection, still flip the packages so anything touching the grills still gets irradiated)

Your bulb might produce some ozone, if it does this would help some by putting ozone into the air, but I would put your things on a rack and a small fan to circulate, (this would also cool the bulb). If it does produce ozone (that smell you might recognize from laser printers or copy machines or a thunderstorm, sweet fresh air...) then make sure you ventilate and do not inhale that directly the ozone also harms you, especially eyes, throat, lungs.

Another good DIY would be an ozone generator in a box, run it for some minutes (depending on ozone output, I am talking over 5000mg/h not some 6-10mg/h "air freshener") and then let it sit for a few hours.

I would recommend for mail getting some UV glasses https://www.amazon.com/gp/product/B003OBZ64M/ and wearing a protective sunshirt and gloves and just passing your mail by the bulb very close (1 inch or less) for 10-20s each side) see this: https://youtu.be/uEix1Yrcjtw

Be safe, thank you for your post.