Virtual Cart + Free Shipping = shipping cost?

I am THOROUGHLY confused. I have some virtual items - some with download files, some with not. Either way, they trigger the "free shipping" module. Awesome.

Except that it seems to be pulling information from weird places? My $_SESSION['shipping']['cost'] seems to be off. See the images.

Any advice on why this might happen, or where to look to fix it?

I have dozens of modules/modifications installed, but here's some of what I've done that vaguely relate to shipping & checkout
USPS shipping module
UPS shipping module
FedEx shipping module
Modified clone of FedEx shipping module
All shipping modules modified for Optional Insurance calculation
FEC
Additional customer fields
COWOA
Product notes on invoices
Dual Currency mods
Edit Orders
Super Orders
Shipping boxes manager

Check the Known Bug Fixes for v1.5.4 post #16 I think is the one that might fix this for you ...
https://www.zen-cart.com/showthread.php?215694-Known-Bugs-%28and-fixes%29-with-v1-5-4

More testing.
Looks like it's happening when going part way checkout with a physical item in the cart, going back to your cart, removing it, and going to complete checkout. It says "free_free" for the module, but there's still a price there in the session.

Aha. Looks like that did it, Ajeh. I sent my reply before I saw yours, and yup, exact same scenario. On the payment info page, the shipping is ACTUALLY zero now.

Thanks for the update that this was able to fix the problem ...