Offer is valid until Oct 7th and the it says that it is available either 1) in store or 2) store pick up.
Apple iPhone 5c 112173 - Best Buy
So I bet you could order it before the 7th online for store pick up with the $50 promo and pick it up at a later date. I don't know, you might have to pick it up by the 7th but what happens if you ordered a color they didn't have in stock? Surely they'd still give you the $50 promo and you'd have to wait for delivery.
I'm not sure if the terms and conditions link will work but it is on the Best Buy website as well:
iPhone
The other thing that is not clear is how you get the gift card, because for the Samsung phone $50 promo it looks like they email you the gift card, but for the iPhone 5c it is not clear how you get it. I think if one went through all the steps of buying online with store pickup it would then become clear at check out, otherwise I'd give them a call before I finalized the buy.