Just my 2 cents worth.
In your case it sounds like they used a USPS box, which I think USPS provides free of charge. I get small & medium flat rate boxes from them at no charge.
The 1st year that I had a daily UPS pickup service I charged what the UPS chart showed. That year I lost about $900 on shipping when we looked at the books come tax time. At that time I think UPS charged me $25 to come by once a day to pick up packages. After the 1st package that dropped by $2, After $50 worth of packages I think it dropped another 2. Some weeks I may not have had but 3 or 5 packages. For the week ending
Dec 24, 22 UPS billed me $632.33 for the packages I sent out. $19.50 of that was my weekly pickup fee & I shipped 13 packages. The losing $ thing was not happening again. Now I charge a flat $4 fee over what the UPS calculator says to cover UPS pickup fees, tape, staples and boxes. I just spent $987.13 dollars on boxes in September, but that might last me a year. That purchase would have cost another $150 -200 without the promotion they had going at the time. Then I have custom boxes made by another vendor also. Not trying to justify their charges, but having a "shipping" department is a burden in materials costs, not to mention the labor costs.
Not wanting to offend anyone, but if your in a business that does daily shipping you know what I am talking about.
Just saying.
Jeff