Cal,
Sorry, it was not aimed at you. Guaranty companies simply do the math. In other words, they take the amount of monetary loss expected by the merchant and then they charge a fee per transaction that covers that plus a bit more. It's risk analysis.
Scanning checks to get the MICR data and then doing a real-time check via StarCheck, SafeCheck, etc. is exactly what they are doing. We can do the same thing. Like I said, it is all about the throughput.
The only difference in your illustration is that the ABA and account number are captured at the point of sale. The MICR data is scanned / read and there is no room for error (ie leading and trailing zeros, check numbers, etc) like there is on the web.
There are also rules on what can be done at the POS and in non face-to-face enviornments (similar to credit cards). In other words, check guaranty in a non face-to-face enviornment is more expensive than the retail POS.
Trust me when I say that if I or any other company is going to guaranty your checks, we are going to charge you in a way that we are not going to lose money.