I found posts stating that these types of issues are usually due to either:
- files not being correctly uploaded, or
- leaving old files on the server.

I compared the modified files from the SbA v4.7a with what I had on the server and found a couple of discrepancies between them. The discrepancies weren't in any of the code that was marked as part of the SbA edits, but none the less when I corrected those discrepancies the store is no longer allowing me to check out with items that are out of stock.