I still have the manual for my BJC-600e, which states the same as yours about not turning it over.
The only thing I can suggest is that you clearly marked the box in which it is being sent "This Way Up Only" and, perhaps, something on the lines of Fragile, Do Not Drop".
Also some large red arrows making clear the box should be kept upright may help.
I did send mine back to Canon's third party official repair centre late last year but, as the cost of a new print head was more than the cost of a new printer, I told them not to bother repairing it.
I'm still very doubtful that the print head was the cause of the problem; what's more, the manual states it should last the life of the printer.
The Canon proved a superbly reliable, remarkably quiet and vibration free printer and sat on top of my system case for convenient use. It was also fully equipped for photo printing...:-)
Now I have an Epson R300, although I would have got another Canon if there had been a suitable replacement.