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Still waiting for the answer to the interchange fee question? Does anyone know?

You are not. InterNACHI’s program is not the same as the Signature one you are using. We negotiated an absolutely untouchable deal for members only: http://www.nachi.org/free-merchant-account.htm

Interchange fees are the fees that the credit card company (Visa/Amex/etc) charges the merchant account provider (Signature/Square/etc). Some merchant providers bill interchange + some additional percentage. Others bill on a rate schedule, and handle interchange themselves. I believe both Signature and Square do the latter, so in either case, changes in interchange rates shouldn’t immediately affect your bill (although if Visa starts charging more, almost everyone will raise their rates eventually).