Thank you all for your comments.
I had not thought about a Skill Saw. I already have one of those ... Will have to see if it's powerful enough to cut the posts, and if it's worth it to make four cuts per post, times 8 posts. The circular saw I already have, too, but it's heavy and cumbersome and dangerous holding it horizontally to cut the posts off at the ground.
Thanks for the links on the Sawzall. I might see saws in person at Home Depot, then check what better prices I can get on line. I'm not sure I can wield the 7 lb saw at the angle needed to cut down the posts, but will try one out at Home Dept. Thanks for the tips on blades, too.