Supposedly right handed folk should ask for a left zipper and vice versa. I have no idea why, but I do automatically reach across to my left side with my right hand to zip/unzip. If you're planning to mate your bag with another to make a double bag, then one of the bags needs a left zipper and the other a right. (Assuming, of course, that the zippers are compatible!)

REI's 20% off coupon is only for regularly priced items so it wouldn't help you here. There's no dividend on sale items, either. Joining REI would not have helped you with this purchase.

Take good care of this bag (especially storing it uncompressed) and it will last you for many years! Western Mountaineering has a lot of excellent tips for the care and feeding of sleeping bags on their website. Those will work for any down sleeping bag.