I used to use them a lot with UPS at my last job about 12yrs ago. There was a sticker you would fill out for COD's and you could check cash or check which really helped out if had no history with the buyer.
When all was said and done you would either get a check back from UPS (either the buyer’s check or UPS’ If you checked cash only). Unfortunately I don’t remember how long it took to get the money back.
The only pain in the process I remember was that the drivers wanted exact change most of the time so you had to make sure the buyer knew this. Other than that, it wasn’t a big deal shipping it that way.
I hope this helps,
Bill
PS, how about that engine?