Why does it sometimes cost more to ship to Amazon Locker?

I was buying shoes on Zappos and as it is an Amazon company I thought I would try to get the shoes delivered to an Amazon Locker but they didn't have that option.

So I looked at the same exact shoes on Amazon (the price and number of that size remaining is the exact same as on Zappos, so it is the exact same stock). I went to check out with deliver set to a Locker close by but it was $7.75. If I changed deliver to my home address it was free. The shoes were $50, I don't have Prime. I have ordered before to a Locker and didn't cost extra.

Can anyone think of a reason for this? Maybe Lockers are hitting capacity because of the holidays? I figured Lockers might be cheaper for Amazon to deliver to than individual addresses, but maybe the real estate costs outweigh the deliver costs? Or Amazon has discovered customers will pay more for the security of a Locker delivery?

I ended completing order via Zappos.