Oh dear. I'm normally not particularly paranoid about buyers, but that does not sound good at all.

I'm guessing your postal method is by large letter or regular airmail - ie no proof of postage.

I would be very worried that the buyer is just going to do an ebay US claim for item not received, which you will lose because you have no proof of delivery.  The autogenerated names means that they can do single claims with multiple IDs and score lots of free items.

I do hope I'm wrong but honestly that is really sounding all sorts of alarm bells to me.

I would block both IDs (this won't stop them buying with another ID but you will be able to get ebay to help you cancel the transaction if they do so.)

Only safe way to post is via two separate packandtrack parcels at $20.10 each.  

Otherwise cancel the transaction and risk bad feedback

I really hope someone else can think of a non-sinister alternative explanation.