I installed 3.08 in my sandbox; all was looking good overall.

I changed the quantities in the order and clicked on "Update Quantities", without having selected a shipping method yet... and I got:

A critical error has occurred.
'ddlShipping' has a SelectedValue which is invalid because it does not exist in the list of items. Parameter name: value

Please post the steps to recreate the error if you are able to replicate it.

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At your service,
Dave Smith