"My time is too valuable to spend driving around to beg or scavenge used boxes from stores. (I hope yours is too, if you value it properly.) "
Not how its done.
Boxes are picked up during regular errands and shopping trips I am already making.
When I am already shopping in DollarTree and one or more employees happen to be restocking shelves all one has to do is ask "Mind if I take the boxes?". Simple grab the box full of folded boxes they have and put it your cart with your items. If its lot of boxes then grab a second cart.
I've previously asked to get boxes from a Family Dollar store's recycling dumpster 15 years ago. I know the day on which a contractor will pick up their outside cardboard only dumpster so when I am on a regular shopping trip I simply pull my vehicle next to the dumpster and get a whole station wagon or pickup load of boxes at once.
When I regularly shop in Menards for both business and personal items my journey will already take me down a extra wide aisle where they place items to restock along with two wheeled carts for boxes. I don't need to ask at all as these are going back to the compactor. I simply place them the cart I am using or in another cart.
When I am shopping in Wal-Mart I grab the flats off the shelf. These are the cardboard trays than can goods and other are displayed in. These are great for sorting inventory and they work great for shipping as putting two of the same sized boxes together makes a perfect box for books. There are also stockers in the aisles with regular boxes.
Since stopping at these locations also involves picking up boxes for a business the mileage is also deductible.
"Even if that weren't so, the added difficulty of storing at lot of randomly sized boxes"
Rather simple. They are sorted by small, medium, large, and extra large. They are already in a flatted state from the store and stood upright on shelving like books so I can simply grab one.
"matching them to the items I sell"
With three decades of experience that takes nothing more than looking at the boxes on the shelf and obviously grabbing one that is big enough for the item plus packing.
"determining their volume"
I use shipping software to upload items. If the item is shipping USPS there is no reason to even enter the size of the box unless it larger than a cubic foot or exceeds 22" in length. That applies to 99% of items I sell.
I don't need to calculate the volume since I don't offer Cubic rates in the listings. Customers are charged the USPS retail rate, while I pay I commercial rate, with the difference paying the FVF fees. eBay currently does not show Cubic rates in the listing or allow you to print Cubic labels. Customers are simply refunded any large differences between the regular rate and Cubic rate.
If you are putting Cubic prices in your listings then you will have use the maximum possible shipping cost for a particular Tier in the listings since you have no idea where a buyer is located. You will also have to account for eBay's FVF on that Cubic rate by increasing the amount of shipping by the FVF fee or the price of the item. That means every time rates go up you will need to edit your listings.
"figuring out how to pack them securely"
It takes no more time to pack a free box than a boughten one. Since free boxes come a lot more variety of sizes it saves a huge amount of time as I don't have to cut down a box or make a Frankenbox.
With three decades of experience there is nothing to figure out. I can visualize exactly how an item is going to be packed in my brain and then immediately get the materials required. Its all muscle memory at this point. I pack items while watching TV.
"And it's unlikely anyway that stores get many boxes in the sizes I normally use."
That is strange because I have hundreds of them ranging from small ones that you can use as internal packing inside a bubble mailer to very large ones that you can ship steamer trunks in. Dollar Tree tends to carry smaller boxes while other stores like Menards have small to large. Furniture stores have very large boxes that you can use to create custom boxes or used for internal box sheaths / wraps.
"A box that is slightly over 0.2 cu ft must pay the rate for 0.3 cu ft."
They may decide to charge you a regular rate for the box since the Cubic rate is suppose to be a commercial rate product delivered to USPS without any additional labor required by a clerk other than scanning it and placing it in cart for a contractor to pickup. For instance, we got a discount on the hundreds of invoices we sent each day at the wholesaler I worked for as these were all presorted in USPS trays and shipped in bulk.
"I'll explain my question with an example. Consider a box whose nominal dimensions are 9x6x3 inches, and whose nominal volume is 0.09375 cu ft, but whose actual volume for whatever reason is 0.10008 cu ft. I'm asking whether the P.O. will accept that box with postage for 0.1 cu ft, or return it for insufficient postage."
Package verification are primarily automated. I recently had my first package in nearly 25 years selling online get flagged by UPS. Clearly no one checked the actual box but they relied on their automatic system which said the box length and height were wrong. I appealed the decision and won because the item was shipped in a large USPS carton used to ship flat rate boxes to your home. I also told that with so many years of experience I don't make such mistakes, especially with two dimensions on the same box.
This can depend on how well you pack your items. Time and time again I see so called eBay experts on YouTube overpack a box with packing materials. When a box is taped shut it actually rocks back in forth because the bottom and / or top of the box is bulged outward from being overstuffed. That means the box is no longer its official listed height but taller. A box with a 1/4" bulge on the top and bottom would now be 1/2" taller than its specs. So when packing boxes be sure to pack them properly with straight sides and not bulging ones. That is just as important for larger boxes that fall under dimensional rates.
1728 x 0.10 = 172.8
Assuming two dimensions are exact the third one would have the following maximum dimension to still be considered as a Tier 1 package.
172.8 / 6 / 3 = 9.6
172.8 / 9 / 3 = 3.2
172.8 / 9 / 3 = 6.4
Dimensions are rounded down to the nearest 1/4" so the following boxes would be Tier 1:
9 11/16" (9.6875) x 6" x 3"
9" x 6 3/8" (6.375) x 3"
9" x 6" x 3 3/16" (3.1875)"
