Good Customer Service
The customer service people at Direct Home Medical wrote back and told me that they could indeed issue a separate order for the DC cord for my CPAP machine and toss it in with the original order, so that I wouldn't have to pay additional shipping charges. I had feared that the system was so automated that an order couldn't be modified in any way once submitted, but they were able to do it, which is a relief. Now I'll have a backup in case of power failures when I'm in Mehama.