PQ M-code not recognizing empty cell in email field

Because of the way null behaves in performing logical operations, one solution is to re-arrange your statement so that you are checking for the null first.

#"Added Custom" = Table.AddColumn(#"Changed Type", "SalesChannel", each
    if (Text.Trim([email]) = "" or [email] = null) and [StoreName] = "Corporate" then "In-Store"
    else if Text.Contains([email], "amazon") then "Amazon"
    else if Text.Contains([email], "ebay") then "eBay"
    else if [StoreName] = "Event Store Server" then "Event Store"
    else if (Text.Trim([email]) = "" or [email] = null) and [StoreName] = "Corporate" then "In-Store"
    else "Co Web"
        )

The problem is that a statement like:

Text.Contains([email], "amazon")

will return null and not a logical if [email] contains null. By checking for null first in your if statement, you bypass that problem.