Commerce 1.2.9 - hotfix
The last Commerce release, 1.2.8, inadvertently introduced an issue in the cart. When changing the quantity before hitting the checkout button, the change is not properly persisted and the checkout will revert back to the number of items that were chosen earlier.
To resolve that we released 1.2.9 today, as well as a 1.3.0-dev6 developer snapshot for people who may be previewing the 1.3 branch.