I have yet to find any lookup or validation software with a suitable interface to allow it to be seamlessly integrated into custom software. I have been repeatedly screwed in the ear by vendors promising such an interface if I would buy the product, then not delivering. You end up cutting and pasting to a separate application both to supply the address and to get the resulting postal code — very clumsy. The post office has an online lookup service. That is probably your best bet for seamless integration.
Digimap offers a list of the legal FSAs (Forward Sortation Areas) in a text file for This can be used for data validation, but not automatic postal code lookup.
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