First off I want to apologize for our tech support guys not getting this taken care off promptly and to your satisfaction.
We have found that the DPMS magazine doesn't drop free from any of our lowers, the M3 20LR PMag has one wide spot that drags on the sides of the magazine well, we don't yet know if this normal for a M3 20LR or not(its the first M3 LR20 we've seen). We did find that the M3 10LR magazines work great and drop free from your lower.
What I'd like to do is swap your 2 magazines with new Magpul M3 10LR PMags and get your lower back to you. This will save you the transfer fee, get you a correctly functioning lower/magazine combo, and avoid the sticky situation with the 20rd magazine. Our tech support guys should getting in contact with you today to sort out the details.