I think its best if you can get the money off people first. the problem is I've done it a couple of times now and havent made people pay upfront - the first time someone new to group ordered stuff and was never seen again (only 2 packs so we managed to offload to others. The next time, someone who i didnt really know ordered loads (about 8 packs) I put the order in but was sweating worrying she wouldnt turn up - thankfully she did.
I'm just waiting to see if someone else volunteers now, because frankly its not worth the hassel - I dont think people appreciate the effort and risk you take in doing these things for people.