I have purchased several items through mileage Plus mall over the last 3 weeks. No miles have posted for any of these purchases. If my miles are currently set to expire Dec. 31, do I lose them if the mall purchase miles don't post by then? Or does United have some way of accounting for that there has been some activity on my account even though the miles have not been credited yet?

And what type of purchases, redemptions (small increments, say less than 2000 miles) are known to post quickly (within a few days) consistently?