Invoice numbers cannot be changed; they are a universal unique identifier for all orders passing through our system, and since ours is a centrally-managed system shared in common among all 10,000+ sellers subscribed to our service, it's not possible to modify how our system operates for just one seller without applying that same modification to every other seller.
For checkouts using Authorize.Net, we simply display as-is any errors their system may generate, which is crucial for troubleshooting any problems in your setup or account settings. "Declined" is industry-standard terminology for when the card issuer refuses authorization for payment, so if a buyer contacts you with any confusion about that particular error message, they're probably just hoping there's some outside chance it doesn't actually mean what they're pretty sure it must mean. You may wish to raise your concerns with Authorize.Net and suggest that their system could generate a more clearly descriptive error in this case.