How do I filter on a post code

We have a shipping agent service cost table and each record has a FROM Post Code and a TO PostCode
For example

No FROM TO charge
1 AA01 AZ99 100
2 ZA01 ZA99 200

Now logically I know that if I pass a post code of AB9 then I want to return record 1 and the charge of 100. However, how do I do that within a filter ?
I know that the issue is the mixture of Text and Numbers in the code field. I don't know how to get round it....

Any help is gladly accepted

Mat