Note that you can have ranges of only one zip code and use the post code prefix and suffix fields when you have alphabetic characters so you should still be able to use that method, even though your method is fine too.
Could you give an example of this? I'd like to offer free shipping for a time to postal codes in the range V6A to V6G but I haven't been able to get it to work. It sounds like this is what I should be setting from the description above.
Prefix: V
Minimum post code: 6
Maximum post code: 6
Suffix: G
PS Rockin' great cart, lots of functionality and yet fairly simple implementation.