Hey the post above mine has really good ideas!
Oh man......I really feel for you. I was a wreck while my babies were in transit and I was called by 10:00 the next day. My postal workers were thrilled to have the chicks with them and didn't want me to take them home! They were kept right up front at the counter where they could be "supervised". From other posts I've read, there are ALOT of really really good postal workers out there. Let's hope yours are somewhere cool and safe with a concerned worker watching over them. One thing I've learned in my short time on this site--There is still hope. Why don't you do a search now for tips on what to have handy to rehab hungry/dehydrated chicks? I've heard pedialyte mentioned. If you have to pick them up perhaps you'll want to take a care kit with you with food and water? Maybe post the topic under the "sick/emergency" heading for expert pointers, I'm a newbie so I'm not exactly sure what to recommend.
PLEASE let us know what happens.
Good Luck, KimberlyJ