Somebody is yanking your chain and they are wrong. See the following directly off the USPS web site. In addition, read the domestic mail manual and there is no exclusion. In fact the following specifically says MONEY, CASH, CURRENCY...click the link to go straight to it, it is easier to read in the chart...

Insured mail - determining replacement costs
Maximum Replacement costs for Insured Mail Articles Item Maximum Replacement Costs
Merchandise Market Value or Cost
Money / Cash / Currency Full value
Nonvaluables - Matter not having intrinsic value such as letters, files, records, etc. Considered to have no value. If postal insurance coverage is purchased, the customer will only be reimbursed for the replacement cost of the documents (See Additional Notes on Indemnity Limits).
Negotiable Instruments (Documents) - Instruments payable to bearer, including stock certificates and money orders endorsed in blank. Market value (all values based on value at time of mailing).
Nonnegotiable Instruments (Documents) - Registered bonds, checks, money orders and drafts made out to specific recipients; deeds, wills, and similar documents. Stock certificates are considered nonnegotiable unless endorsed in blank. Considered to have no value. If postal insurance coverage is purchased, the customer will only be reimbursed for the replacement cost of the documents (See Additional Notes on Indemnity Limits).
Jewelry, Gems, Precious Metals Market value or cost