Let's say each MoC gets 200 letters per day -- a reasonable estimate based on some quick Googling. 535 members times 200 letters equals 107000 pieces of mail per day.
Suppose you pay some worker minimum wage to screen mail. They spend on average 20 seconds examining each piece of mail. That's 594 man-hours per day. Minimum wage in DC is $8.25 per hour. So, $4900 per day to screen the mail, just for labor costs.
$4900 per day is a pretty solid base estimate. On top of that, there are costs associated with enhanced checking for "suspicious" items. Assume 1 in 1000 items is deemed suspicious and undergoes extensive chemical testing at a cost of $50 (that's being generous). That's $5350 additional per day. A total of $10,250 per day to check the mail. 52 weeks a year, 6 days a week of mail is 312 days per year. Total cost per year is therefore $3.2 million.
Don't you think such an expenditure is completely idiotic? For one thing, the system can fail, despite all your checks. Something could slip through. On the other hand, you could, for a small fraction of that money, design and implement a robotic system which automatically opens the mail, digitally scans it, and transmits it to the MoC in the form of a PDF. 0% chance of failure, as well as much much cheaper.