I can't make that call...that's up to you. I would think they'd be fine, as long as the pad is low enough and you don't have to worry about it turning itself off during the night. It looks from the picture that your cave is at a good height, so if that pad is about back level they should be plenty warm. Remember that mine went from the feed store to the car to the brooder in the run, and it was 21 degrees out there.
Just so you know, you might lose one or even two to shipping stress if they were that delayed in the mail, so it might be better to have them in the house where you can keep an eye on them for the first 24 hours or so, just so you can act quickly if any show problems. But after that, the garage should be fine. What a cute passel of babies!
Edited to add: I also see you are using the tote lids.....good thinking! Even though I brood mine outside, you can bet that's an idea I'm stealing too! I thought it was so smart when I saw that posted....I think it was aart who showed that!