The only reason they couldn't open the box and put more stuff into it is if they don't know how. I do it all the time when I get multiple orders from someone for pens.
You just repackage, use the priority shipping label and charge the customer for both shipping labels. I always include a written explanation why they are paying for two shipping charges, i.e., because I had to pay for two shipping labels, but I wanted them to get both orders as quickly as possible. I've never had a customer complain about it. I'm just glad for the additional purchase. I would never alienate a customer like Penn State did by refusing. So I know it can be done. It all boils down to customer service, and lately Penn State doesn't even know how to spell those words, let alone exhibit them. I refuse to buy anything from them any longer after the rude treatment I got from them multiple times last year. I was a customer of their's for over 8 years but no more. I even bought my first lathe from them. Things really went south with them when they expanded. Their product quality is still okay, but they show no desire to maintain good customer relations anymore. They don't realize that their total sales are just a total of the sales they make to individual customers.
Every customer is vital to survival, no matter how big you get. Look at the US Auto makers for an example. Penn State is just too dam* big for their britches these days.
When an order comes in, it goes to picking then packing and then to the shipping department usually within a few hours. Once it hits shipping the labels are created and electronic data is sent to USPS or UPS. They just cannot open a box packaged the day before and stuff more stuff in it.
Yes, rudeness is intolerable, but you are asking for something that cannot be done. Don't take it out on the poor person who answers the phone. She is either an order taker or a customer service rep and she cannot change or affect policy and procedures.